What happens when hyperintelligent roommates Sheldon and Leonard meet Penny, a free-spirited beauty moving in next door, and realize they know next to nothing about life outside of the lab. Rounding out the crew are the smarmy Wolowitz, who thinks he's as sexy as he is brainy, and Koothrappali, who suffers from an inability to speak in the presence of a woman.
Sheldon and Leonard must divide up their possessions and remove Penny's things from 4B.
Amy's desire to have an intimate night with Sheldon on her birthday is interrupted by Bernadette going into labor.
Sheldon and Amy visit Leonard and Penny for the first time since the holidays and share details about their terrible trip to Texas.
Leonard and Penny may need a Relationship Agreement, and Sheldon volunteers to prepare one.
Sheldon uses a machine to help him identify other people's emotions, and Raj tries to understand his difficulties with women.
Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system. Also, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley.
Sheldon and Amy have their first fight since moving in together and Koothrappali is shocked at the realities of financial independence when he stops receiving support from his father.
The guys' annual trip to Comic Con is in doubt when Raj can't afford to go.
Raj searches for a cheaper living arrangement.
Leonard, Penny and Raj adjust to their new living arrangement and Sheldon takes an interest in Amy's work.
Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Wolowitz as well as Amy on the same day. Also, Bernadette is nearing the end of her maternity leave and must return to work.
Howard and Bernadette have a hard time leaving Halley in day care when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.
Leonard isn’t thrilled when Penny considers taking a new job offered by her ex-boyfriend, Zack.
Amy is offered a Fellowship at Princeton, Raj makes new living arrangements, and the gyroscope project is spinning out of control!
The gang is concerned when Sheldon’s former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.
When Bert, a Caltech geologist, wins the MacArthur Genius fellowship, Sheldon is overcome with jealousy. Also, Wolowitz finds an old remote control Stephen Hawking action figure he invented.
After a successful experiment combining their genes, Sheldon will stop at nothing to convince Amy they should procreate. Also, Koothrappali is embarrassed to tell the gang what the new woman he’s dating does for a living.
In order to continue living with Sheldon, Amy lies to him about repairs being made to her apartment and they host an "on location" episode of "Fun With Flags" in Penny’s apartment. Also, Leonard lets Penny decorate Sheldon’s old room after he learns that she has been secretly moving Leonard’s collectibles into storage.
Penny is surprised to learn she has fans of her performance in "Serial Ape-ist" when she attends Van Nuys Comic Con with Leonard. Also, Amy hosts a Sheldon-style brunch at Penny’s apartment, and Wolowitz makes an impulsive purchase when he starts stressing out about the baby on the way.
Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation.
When Amy's apartment floods, she proposes a "cohabitation experiment" with Sheldon. Also, Howard and Bernadette are upset when Koothrappali learns the gender of the baby before they do.
Sheldon can't keep up with the late nights Howard and Leonard are putting in, Raj spends the day with a pregnant Bernadette and Penny and Amy attend a faculty party.
The Air Force shows interest in the guidance system and Penny lets it slip at work that Bernadette is expecting.
After Leonard's father, Alfred and Sheldon's mother, Mary share an evening together, everyone deals with an awkward morning the following day. Also, Penny's family arrives for the ceremony, including her anxiety-ridden mother, Susan, and her drug dealing brother, Randall.
Amy enthusiastically accepts Sheldon's marriage proposal. However, when dining with Amy's colleagues later that night, Sheldon is offended that they are more impressed with Amy's work than his own. With help from Stephen Hawking, Sheldon later comes to grips with the fact that he will not always be the center of attention in the marriage. Bernadette is shocked to realize that she is pregnant again. She and Howard are not thrilled at the idea of having another baby so soon after their first. They attempt to convince Leonard and Penny to have a child as well, but are rebuffed.
Leonard gives an interview on public radio where he admits physics research might be at a dead end. The university is furious and, while trying to think of reasons to put in a retraction, he depresses Sheldon into thinking he might be right. Howard and Raj come over and end up just as depressed. Getting drunk with Penny, the men go to the grave of Richard Feynman and realize there is hope for physics as long as they believe in it. Leonard accidentally sends a drunken email to Human Resources. Meanwhile, Amy and Bernadette agree with each other to keep quiet about the success in their careers so as not to upset Sheldon and Howard. While bragging to each other, they end up arguing which of their fields is better.
Sheldon cannot find the perfect wedding date and begins talking in his sleep, adopting the persona of a laid-back person who goes with the flow. Amy and Penny believe that his subconscious mind is trying to tell him to be more relaxed; his attempt to be so while wearing flip-flops ends in disaster. He decides to leave all the wedding planning to Amy while he wears a Star Trek uniform underneath his tuxedo. Meanwhile, Raj and Stuart both become interested in Bernadette's new coworker Ruchi. While she likes them both as friends, she does not want a relationship with anyone.
Howard and Bernadette learn their second child will be a boy, leading Howard to doubt whether he can be a good father to a son. He and Sheldon go out in the desert to test launch a model rocket, but it explodes, causing him further doubt. On the way home, Howard does a good job coaching Sheldon to drive them home, for which Sheldon tells him he will be a good teacher to his son. Leonard's mother Beverly begins talking to Penny as a friend and confidant, troubling Leonard, especially when he learns that Beverly told Penny that she was proud of her, a level of praise she never bestowed upon Leonard himself. When Leonard confronts Beverly, he is touched when she says that of all of her children's spouses, Penny is the one by whom she is most impressed and that, for this, she is indeed proud of him. Meanwhile, Raj helps Bernadette turn baby girl clothes into boy ones.
Amy and Howard begin collaborating on a neuroprosthetics project, upsetting Sheldon and Raj, who feel they are not spending as much time with them. When they seek solace with an annoyed Bernadette, she exploits the situation to trick Sheldon into doing Howard's chores. Penny employs the approaches recommended in one of Bernadette's parenting books to deal with Sheldon with success, but Leonard believes she is coddling Sheldon too much. Sheldon and Raj resolve the issue of missing their significant others by spending time together.
Sheldon's childhood science show with Professor Proton is being rebooted. After unsuccessfully auditioning for the part, he goes to Wil Wheaton for acting tips for his second audition. However, Sheldon is later dismayed when Wil has somehow taken the role instead. Talking it over with Arthur in his dreams he is still upset his idol is replaced and considers Wil an enemy again. Meanwhile, Howard gets a vasectomy while Bernadette is on bed rest. Penny offers to take care of Halley for them but they insult her by saying Penny is irresponsible. Halley ends up saying her first word by calling Penny "Mama."
Bert asks for Sheldon's help in a geology research project relating to dark matter. Sheldon secretly agrees, but is too embarrassed to admit he is working with rocks. Bert finds out about this and ends their partnership. After talking with Amy, Sheldon goes to apologize, only to find Leonard has taken his place. Meanwhile, Raj runs into Ruchi again and goes out with her, but she does not believe in romantic love. After Howard and Bernadette tell Raj that he can just have sex with Ruchi, he agrees to keep it casual.
After learning Sheldon went behind their backs to work with the military again, Leonard and Howard say Sheldon is like Thomas Edison, taking credit for other's ideas, while they are more like Nikola Tesla. Leonard and Howard bring in Barry Kripke to help create a better idea than Sheldon's, but Kripke goes to the military himself cutting all three of them out. Meanwhile, Bernadette fears Ruchi is trying to steal her projects at work while she is on maternity leave, and hires Raj to spy on her. Though he finds proof that Ruchi may be stealing from Bernadette, Raj tries to defend Ruchi like a boyfriend would, pointing out Bernadette's hypocrisy as she would've most likely stolen from other people's projects as well. Ruchi is not happy about this and promptly dumps Raj.
In 2010, Leonard, Howard and Raj mined a lot of Bitcoin, worth thousands of dollars seven years later, but leave Sheldon out due to his fear of tax implications. Flashbacks are shown of how the Bitcoin ended up on an old laptop of Leonard's, which Penny gave to her old boyfriend Zack after she and Leonard broke up. In 2017, Leonard and Penny retrieve the computer from Zack, who first shows them a video of a drunken Penny apologizing for breaking up with Leonard. The Bitcoin is not found on the laptop. Sheldon had moved it to a flash drive on Leonard's keychain to teach them a lesson, but Leonard lost the keychain years ago. A final flashback to 2013 shows Stuart discovering the flash drive, which he decides to erase and sell for $10.
When Raj loses a job interview at a planetarium, his father says he has no confidence because Howard always makes fun of him. Raj distances himself from Howard and later gets the job. Howard is hurt by this (even after apologizing to him) and tries to make amends with Raj after his first talk at the planetarium, but when he sees him get a date, he leaves without speaking to him, thinking that the latter's life is better without him. Meanwhile, Sheldon and Amy try randomly dividing up wedding tasks, but they keep arguing. The only thing they can agree on is to get married so they decide to just get married at City Hall. However, Sheldon decides that he wants a first dance with his new wife, and they go home to plan their wedding.
Howard and Raj are still fighting so Howard dis-invites Raj to Halley's birthday party. However, when Howard's designated party planner Stuart pulls out at the last minute, Raj agrees to plan the party, which doesn't turn out too well due to Halley being asleep, Bernadette on bed rest and no people turning up. When Howard and Raj get into a shoving match in the party's bounce house, they end up letting out their frustrations with each other, but also having fun and reconciling. Leonard is depressed upon receiving his successful brother's Christmas card and attempts to make a list of what he and Penny accomplished this year, but doesn't come up with much. This prompts them to think about their plans for the future, so they decide to go on trips and achieve more out of life together before starting a family. Sheldon cooks Amy a Little House on the Prairie-themed birthday dinner, which gives them both food poisoning. They eventually recover enough to attend Halley's birthday party with the others, where they decide to have fun in the bouncy castle and then have sex in Howard and Bernadette's house.
To discover who would be most qualified to be best man and maid of honor at their wedding, Sheldon and Amy subject their friends to a series of secret experiments. Also, Penny reveals her true feelings about Amy.
When Sheldon kicks Amy out to work solo, she and Leonard bond during a series of science experiments. Also, Bert the geologist replaces Wolowitz in the band Footprints on the Moon, and Sheldon finds Penny a surprising source of scientific inspiration.
Koothrappali finds himself in the middle of domestic drama when he learns the woman he's dating, Nell, has a very upset husband, Oliver. Also, when Sheldon rents his old room back for a quiet place to work, he drives Leonard crazy by being a model tenant.
When Sheldon petitions Wil Wheaton to appear on the new Professor Proton show, Wil offers a role to Amy instead. Also, Penny gets angry when she thinks the female protagonist in Leonard's novel is based on her.
When Bernadette won't go into labor, all her friends try different tactics to get things started. Also, Wolowitz confronts Bernadette after Amy accidentally reveals she's already chosen their son's name.
Leonard jumps through hoops to help secure the perfect wedding venue for Sheldon and Amy. Also, Bernadette and Wolowitz have a hard time deciding who should stay at home with the kids and who should go back to work.
Penny gets the opportunity to host Bill Gates at work when his company wants to partner with her pharmaceutical company. Also, Leonard, Koothrappali and Wolowitz do everything in their power to meet him, while Sheldon thinks he is the victim of an April Fools prank.
When Leonard learns Sheldon is the president of the tenants association, he decides to run against him. Also, Bernadette encourages Wolowitz and Koothrappali to find the owner of a drone after they find it in the backyard.
Sheldon takes the guys to a cabin in the middle of nowhere to meet a reclusive scientist; Penny and Bernadette improvise Amy's bachelorette party.
Sheldon’s comic book store experiences changes when writer Neil Gaiman puts Stuart’s store on the map. Also, Koothrappali takes credit for Penny’s astronomical discovery, and friendships are threatened.
Sheldon goes to Vegas to win money for science. Also, Penny and Bernadette take Amy wedding dress shopping, but her terrible choice entangles them in a web of lies.
When Sheldon learns that his mother won’t attend his wedding unless he invites his brother, Georgie, he and Leonard travel to Texas to end a family feud. Also, the Wolowitz kids inadvertently infects Amy, Bernadette, Wolowitz and Koothrappali with pinkeye.
Leonard and Penny get remarried in front of their friends (who show up), Leonard's parents, Beverly and Alfred (Judd Hirsch) and Sheldon's mother, Mary, who finds something in common with Alfred. Howard gets an unknown email from the Air Force about his invention.
Sheldon hires Stuart to spend the day with Amy when he’d rather go to a movie screening. Also, Leonard’s mother, Beverly, comes to town and Penny struggles to make a connection with her.
Sheldon and Bernadette spend a night together while the rest of the gang goes to a wine tasting party, but things get uncomfortable for Raj when Zack also attends and takes an interest in Claire.
Everyone must choose sides when a small argument between Leonard and Sheldon erupts into a heated fight during a group get-together.
Secrets are revealed when Amy and Sheldon join Penny and Leonard on a weekend cabin trip. Raj is more excited about Howard's and Bernadette's impending parenthood than they are.
Amy is shocked at a revelation from Sheldon after she buys him a new laptop. Koothrappali sells out Leonard and Wolowitz after they lie to their wives in order to attend an early screening of a movie.
After more than nine years together, the gang finally celebrates Sheldon’s birthday, surprising him with a special guest.
Sheldon works up a contract to split their portion of their guidance system invention royalties, but Bernadette does not like how it's done. Raj must again decide between Emily and Claire when they both contact him.
When the gang finds out about Bernadette's pregnancy, they celebrate by singing karaoke. Also, Bernadette is concerned when Wolowitz freaks out over her pregnancy announcement, and Wolowitz is convinced he must make more money to provide for a baby.
Sheldon and Amy host a live Valentine's Day episode of Fun with Flags, Leonard and Penny grapple with getting older, Howard and Bernadette find a surprise in their hot tub and Raj is torn between Emily and the cute girl he met at the Comic Bookstore.
Sheldon is thrilled when his Meemaw comes to visit, but his excitement quickly fades when she butts heads with Amy. Also, Raj meets a new girl at the comic book store who makes him question his relationship with Emily.
The gang decides to head to Vegas for the week on a party bus. Sheldon learns about the importance of empathy and saying sorry.
Leonard visits a psychiatrist so Penny can make a sales call. Stuart moves out of the Wolowitz house. Sheldon works with Raj looking for extra-solar planets.
As the guys prepare to go to the premiere of the new Star Wars movie, Sheldon plans a special event for Amy's birthday.
Sheldon has a revelation when he realizes why a song was stuck in his head. Also, Amy invites Dave over for dinner despite their previously awkward date, and Wolowitz and Koothrappali become obsessed with a fan of their band, "Footprints on the Moon".
It’s Thanksgiving. Sheldon and Amy go on a lunch date to the aquarium as friends. Howard, Bernadette, Raj and Emily help out at a soup kitchen. Leonard and Penny make their first Thanksgiving dinner as a married couple.
Amy has a formal date which Leonard, Penny and Bernadette secretly spy on. Sheldon has Howard and Raj help him find a new girlfriend since they found Amy for him.
Adam Nimoy, friend of Wil Wheaton and son of Leonard Nimoy, interviews Sheldon for a Star Trek documentary. Sheldon gets off topic and ends up revealing an engagement ring and his plan to propose to Amy. Bernadette wants to start renovating the house and the issue of starting a family with Howard is brought up.
In the midst of a nation-wide helium shortage, Sheldon and Leonard take desperate measures to get the supplies they need. Also, Penny and Bernadette download a dating app on Amy's phone to try and find her a new man.
Leonard and Penny finally move in together leaving Sheldon very upset with this big change in his life and wishing he could go back to a time when he had no emotions in 2003 before he met Leonard. Raj, Howard and Stuart attempt to form a band.
The guys take a fencing lesson from Barry Kripke and Sheldon finds himself in an awkward position when Kripke expresses interest in Amy. Also, when Stuart struggles to get female customers at the Comic Book Store, Penny, Amy and Bernadette come by to help him out.
The guys’ science skills are put to the test when they get a flat tire during Leonard’s bachelor party weekend in Mexico. Also, the girls force Penny to finally tell her family that she eloped with Leonard.
Leonard confronts the woman he kissed on the North Sea in an attempt to set Penny’s mind at ease. Also, Sheldon films a special episode of “Fun with Flags” after his breakup with Amy, and Bernadette feels guilty about withholding a secret from Penny.
Sheldon is confronted by a mystery of the universe he cannot unravel: when a woman wants time apart to think, exactly how much time does that mean, and is there any way to hurry the process along? In Las Vegas, Penny and Leonard march closer to marching down the aisle, but has Penny gotten over Leonard’s infidelity? And if so, will he do anything to un-get her over it?
Sheldon must choose between joining Amy at her aunt's birthday party or playing videogames all weekend with the guys.
When Wolowitz gets to work with Stephen Hawking, Sheldon is willing to do anything to meet his hero.
After the guys throw Howard a bachelor party, Bernadette reconsiders marrying him after learning about his sexual history.
When NASA moves up Howard's mission, he must face his fears and Bernadette's dad, thus putting the wedding plans in jeopardy. Meanwhile, Leonard says something surprising to Penny in the bedroom.
The fifth season ends with Howard and Bernadette deciding to get married before his NASA mission, causing the gang to scurry into overdrive in an effort to stage the wedding. NASA astronaut Mike Massimino has a cameo as himself.
Sheldon tries to intervene when he learns that Penny is thinking about breaking up with Leonard; Howard is picked on.
Amy feels threatened by Sheldon's new assistant; Wolowitz starts to feel the pressures of being in space.
Wolowitz's return from space isn't all he expected; game night becomes a gender war.
The gang heads to Stuart's comic book store to celebrate Halloween; Leonard uses science to seduce Penny.
A game threatens Sheldon's relationship with Stephen Hawking; Penny secretly signs up for a college class.
Bernadette wants Wolowitz to move out of his mom's house; Sheldon is caught in the middle of Amy and Wil Wheaton's argument.
Sheldon and Wolowitz's fight over a parking spot at the university impacts the whole gang.
When Amy gets sick, Sheldon tries to nurse her back to health; Wolowitz and his father-in-law plan a fishing trip.
During a game, Sheldon revisits Christmas memories; Amy, Penny and Bernadette look for a date for Koothrappali.
When Sheldon is accused of sexual harassment, he ends up getting Raj, Leonard and Wolowitz in trouble.
As the guys don costumes and go on a road trip to a comic book convention, the ladies decide to stay home and try to interpret a comic book.
Sheldon faces a crisis of confidence when he is forced to work with his nemesis, Barry Kripke; Raj and Howard buy expensive action figures of themselves.
A fight between Sheldon and Leonard impacts Amy and Penny's living arrangements; while Howard is away, Raj helps Mrs. Wolowitz out.
The guys search for a perfect Valentine's Day gift for their significant others; Stuart and Raj host a "lonely people" party at the comic book store.
After a bad date, Koothrappali decides to never leave his apartment; Penny's acting skills impress Sheldon.
Leonard, Sheldon and Wolowitz talk to junior high girls about science-related careers; Raj plans a date with Lucy.
Howard considers whether or not he should open a letter from his father; Penny and Leonard host a "grown-up" cocktail party.
The girls get involved when Leonard, Sheldon and Raj compete for tenure at the University.
Amy tries to help Sheldon get over his compulsive need for closure when one of his favorite TV shows is canceled; Raj discovers a secret about Lucy.
Leonard and Sheldon hire the host (Bob Newhart) of their favorite childhood TV show to perform; Bernadette and Howard run into trouble while dog-sitting for Raj's pet.
Amy and Sheldon's relationship takes a surprising turn during a game of "Dungeons & Dragons"; Lucy and Koothrappali have an awkward date.
When Leonard is considered for an exciting research opportunity, Sheldon becomes jealous and tries to scare him into backing out. Meanwhile, Raj's relationship with Lucy is strained when he tries to get her to meet his friends.
Raj faces his loneliness when his friends are out on dates; while in space, Wolowitz gets drawn into a fight between Bernadette and his mom.
Raj and Howard try to to figure out why Sheldon disappears at 2:45 every afternoon; jealousy arises in Penny and Leonard's relationship.
In the first part of the one-hour seventh season premiere, Sheldon and Penny bond in Leonard's absence, but it's Sheldon's feelings that are crushed when Leonard returns from the North Sea. Guest star Regina King returns as Mrs. Davis, the University's Director of Employee Relations.
On the second part of the one-hour seventh season premiere, Raj gets consoled about his ex-girlfriend and Howard's relationship with his mother causes an unusual threat to his masculinity. Kevin Sussman returns as Stuart, the comic book store owner.
When the entire gang competes in a cutthroat scavenger hunt designed by Raj that tests their science knowledge and street smarts, it brings out the best – and worst – in everyone.
After Amy ruins one of Sheldon's favorite movies – Raiders of the Lost Ark – he gets defensive, seeking revenge. Meanwhile, Leonard discovers a way to get Penny to do anything; and Raj and Stuart create online dating profiles.
Sheldon must decide how much "Amy time" is too much after she takes a job at his university, and their relationship is put to the test. Meanwhile, Howard finds himself sleeping on Raj's couch after a fight with Bernadette.
Sheldon's latest scientific breakthrough makes him feel like a fraud – and threatens to haunt him forever. Meanwhile, Howard's romantic gesture to Bernadette causes Penny to step up her game with Leonard.
Sheldon feels slighted when Professor Proton (recurring guest star Bob Newhart) seeks advice from Leonard instead of him, and he seeks revenge by befriending a rival science TV host, Bill Nye (guest starring as himself). Meanwhile, Raj gets jealous when Howard crashes "girls' night."
Leonard tries to keep Sheldon from overreacting when a past mistake comes to light, but Sheldon punishes Leonard by making him "walk a mile in his shoes." Meanwhile, Penny confronts Raj's ex-girlfriend, Lucy (guest star Kate Micucci).
An upset Sheldon protests when the entire gang drags him to Mrs. Wolowitz's house for Thanksgiving. Meanwhile, Penny and Leonard must deal with a mistake from her past.
Sheldon faces a crisis after Leonard disproves his accidental scientific discovery. Meanwhile, Raj must stay with Howard & Bernadette for a week.
While Sheldon is in Texas, the rest of the gang realize how much he has changed their lives as they decorate the apartment Christmas tree.
As Leonard tries to help Penny with a disappointment, Penny asks him a bold question about their relationship; Sheldon wants to learn how to be funny.
Sheldon must learn how to relax when he is forced to take a vacation; Leonard tries to support Penny when she quits her job; Bernadette needs to replace one of Stuart's comic books.
Unable to get Comic-Con tickets, Sheldon tries to hold his own convention and ends up spending a wild evening with James Earl Jones; the ladies try to act maturely.
Sheldon and Amy go on a trip to wine country with Howard and Bernadette; Penny and Leonard rush Raj's dog to the vet
When Leonard buys a dining room table, Sheldon re-evaluates the changes in his life; Bernadette is torn when Wolowitz gets the chance to go back to space.
Bernadette tries to negotiate peace between Sheldon and Wolowitz; Penny receives an embarrassing offer; Raj wants Amy to write to a woman on his behalf.
Sheldon is faced with a new reality when he visits his mom in Houston; a murder mystery party starts arguments.
Raj struggles with guilt when he dates two women at once; Penny has a chance to play a part in a cheesy movie; Sheldon is torn between two gaming systems.
Sheldon has a personal crisis when he decides string theory is a waste of his time; a double date ends in embarrassment for Howard.
Sheldon gives spontaneity a try, which causes unexpected friction for Penny, Bernadette and Amy; Raj asks Howard for help with preparation for a date with Emily.
Professor Proton helps Sheldon deal with grief; Leonard gets into a competition with Penny; Bernadette and Amy prepare a "Star Wars Day" gift for the guys.
A bad day compels Penny to re-evaluate her choices -- including Leonard; Bernadette and Howard must care for Mrs. Wolowitz; Sheldon gives Raj advice on relationships.
Faced with an uncertain future, Sheldon considers a major change; Emily and Raj decide to take things to the next level.
Leonard and Amy make an unexpected trip to Arizona to fetch Sheldon; Penny interviews for a job at Bernadette's company; Stuart's relationship with Mrs. Wolowitz disturbs Howard.
When Sheldon must teach a class, Howard decides to take it; Amy takes advantage of the tension between Penny and Bernadette.
Wolowitz feels nervous when he is asked to throw the first pitch at a Los Angeles Angels game; Penny and Leonard want to prove Sheldon wrong after he says he and Amy are the better couple.
Raj's honesty comes back to haunt him when his new girlfriend is unfriendly to Penny; the guys think about becoming part owners in Stuart's store.
During their weekend in Las Vegas, Bernadette and Amy accuse Penny of being a buzz kill; the guys try to invent the next big thing.
Sheldon and Raj test whether they could survive a dark-matter research expedition in a salt mine by simulating the conditions in a steam tunnel deep underground. Leonard and Penny seek advice from Howard and Bernadette after they fight about money.
The guys try to help Penny out of an awkward work dilemma, when a smitten doctor shows up at her door after her flirtatious sales techniques work a little too well. Meanwhile, Amy and Bernadette argue over whether or not female scientists should play up their sexuality.
When the gang recreates a high-school prom night on the roof of the guys'; apartment building, Sheldon feels pressure to participate in all the typical romantic traditions.
When Leonard gets minor surgery on his nose, Sheldon is the one in need of sympathy. Meanwhile, Raj's parents' impending divorce causes Howard and Bernadette to work on their own marriage.
While Sheldon says a tearful goodbye to the final instalment of "Fun with Flags", Leonard, Howard and Raj search for something significant in a dead professor's research. And Bernadette is shocked to learn of her co-workers’ true feelings from Penny.
Leonard and Howard turn to Raj for help in dealing with a work emergency when they nearly come to blows after blaming each other for a potential disaster at the university. Amy throws an authentic Victorian Christmas party, while Sheldon seeks revenge on her for making him celebrate the holidays.
Wolowitz tries to distract Raj, who is anxiously awaiting data from a space probe he helped launch. Meanwhile, Sheldon and Leonard go dress shopping with Penny and Amy.
The gang tries to help Sheldon when he becomes stuck in a rut at work by increasing his anxiety level. Howard teases Raj with a new game called "Cinnamon or Emily?"
Leonard and Sheldon publish a paper which is widely accepted except for comments from an internet troll. Meanwhile, the girls reminisce about embarassing incidents from their past.
Wolowitz receives some shocking news as everyone prepares for the reopening of Stuart’s comic book store. Penny teaches Sheldon how to “let it go,” but can’t follow her own advice after she learns something infuriating about Amy. Also, Leonard and Raj think they spot Nathan Fillion at a restaurant.
As a joke, Sheldon and Penny try an experiment designed to make participants fall in love. Also, Leonard, Amy, Raj and Emily spend the evening trying to escape a room with a “zombie.” And, after returning from Mrs. Wolowitz’ funeral, Bernadette and Howard run into trouble at the airport.
Trouble arises when Amy learns Sheldon has applied for a one-way mission to colonize Mars. Also, a trip to the pet store ends in a fight for Sheldon and Amy, Leonard surprises Penny with a very “adult” purchase, and Raj is caught snooping in Emily’s apartment.
Friction ensues when a magazine fails to mention Leonard in an article about the paper he co-wrote with Sheldon. Also, a blackout at Mrs. Wolowitz’s home leads to a final family dinner in her honor.
When Leonard and Sheldon are invited to speak at UC Berkeley, they take a detour to try to meet an idol. Also, when Bernadette and Howard battle over the fate of his Doctor Who TARDIS, Raj and Amy offer to settle the dispute with a game of ping pong.
When Sheldon and Amy build a blanket fort, they come to major crossroads in their relationship. Also, Wolowitz meets the half-brother he never knew he had, and when Penny is invited to appear on Wil Wheaton’s podcast, she and Leonard have a huge, on-air argument.
When Raj is asked to create a message in case a NASA mission discovers alien life, the guys fight over what he should do. Also, Penny is torn between auditioning for a movie or keeping her successful pharmaceutical sales job.
Wolowitz questions his engineering abilities when he and Sheldon can’t get a toy drone to fly. Meanwhile, a cancelled flight nearly prevents Leonard from giving the commencement address at his former high school, and Raj pits his parents against each other when his father cuts him off financially.
Personalities collide when Sheldon and Leonard’s mothers finally meet. Meanwhile, Howard finally decides to “man up” and do his fair share of the housework – but not without a little help from his friends.
Penny squares off with a new beautiful female neighbor who may become the building's "new Penny."