The kingdom is threatened by an army of Lava Monsters, and the inexperienced NEXO Knights must come to the rescue.
The Cloud of Monstrox and Jestro unlock a mysterious forbidden power; Merlok 2.0 and the Nexo Knights try to make the new Combo Nexo Powers work.
Jestro and the Cloud of Monstrox encounter the sculptor who helps unleash a horde of fierce garden gnomes on Knighton.
Jestro and the Cloud of Monstrox unleashes the Forbidden Power Ravaging Rot on Knighton, turning everything into green gooey rot, but Robin has an awesome new invention that could help the Nexo Knights.
The Nexo Knights have their powerful shields stolen by Robot Hoodlum and his Merry Mechs; something is coming to life in the petrified forest and it doesn't like knights.
Villagers are being enslaved to dig out three ancient warriors whom the Cloud of Monstrox wants to bring back to life.
At K.O.M.A. (Knightonia Official Museum of Art) Monstrox learns that a Forbidden Power tablet is hidden and he will do anything to get it, especially if King Halbert happens to visit at the same time.
The old Lava Monsters are back and a Forbidden Power is in the middle of their new village.
The three Harpies have kidnapped Macy's mother Queen Halbert, and the Nexo Knights use everything in their power to rescue her.
The Nexo Knights attend the Gold is Good Ball hosted by Lance's parents in the extremely wealthy town of Arrindale. Unfortunately, Jestro feeds the Book of Greed to the Book of Monsters to make the Lava Monster Army (who are now led by the newly-summoned General Magmar) so greedy, that they steal all of The Richmond's riches (much to Lance's sorrow) along with both of Lance's parents. However, Lance is not as willing to save his parents as they forced him to become a knight in his childhood while Lance wanted to be a celebrity. Meanwhile, Lance's parents believe that Jestro's Lava Monster Army are servants at a spa and as such are completely oblivious of their imprisonment much to the annoyance of the Book of Monsters and General Magmar. This makes Jestro more than willing to return them and the stolen riches to Arrindale.
The Book of Monsters plans to consume a limited edition copy of a Ned Knightley comic book, which will guarded by the Nexo Knights until it is won at a costume contest at Knight-A-Con (this show's parody of the San Diego Comic Con). Robin is also attending the convention using his newly designed Mini-trex (a miniature version of the Fortrex). When General Magmar gets impatient with Jestro's plan to blend in with the attendees, he launches an all out attack on Knight-A-Con which causes the comic book to get lost in a crowd of terrorized attendees. Fortunately, the Mini-trex is able to extend the range of the Nexo Knights' connection to Merlok 2.0, allowing them to obtain their Nexo Powers and find the book. Because of this, Robin wins first prize at the costume contest and thus, he and the Nexo Knights get to keep the comic.
On King Halbert's birthday, Jorah Tightwad decides to prove that the Tighty Knighties are better than the Nexo Knights at King Halbert's birthday tournament, using the Book of Envy as proof that they encountered Jestro. If the Tighty Knighties win, Macy has to kiss Tightwad. If they lose, Jorah has to kiss Lance's "sweetheart." The next day, the tournament is held to see who are the best group of knights in Knightonia. All four Tighty Knighties best the Nexo Knights in the tournament except Macy who is forced to be her father's princess for the day and thus is forbidden to compete. Before King Halbert can announce the end of the tournament, Jestro and the Book of Monsters lead the Lava Monster Army into attacking the birthday tournament. The Tighty Knighties are then shown being completely useless at fighting them and cowering in fear. At that moment, Tightwad reveals the truth of how he obtained the Book of Envy where it fell into his hands when Merlok scattered the evil spell books in the first episode. This distracts him long enough for Macy to take the book from him. After powering up and defeating the Lava Monster Army, the Nexo Knights are declared the true winners of the tournament while Tightwad is forced to kiss Lance's "sweetheart" on the lips. This turns out to be Hamletta much to Tightwad's horror. King Halbert is proud of what Macy did today, saying there's a time for her to be a princess and a time for when she needs to be a knight. Meanwhile, Jestro and the Book of Monsters have succeeded in stealing the Book of Envy which is soon consumed by the Book of Monsters.
The famed Knighton Chef Gobbleton Rambly has created a chili recipe so spicy, the Book of Monsters gains the idea to weaponize it. Knowing this, Clay has Axl and Chef Eclair compete in a cook off with the grand prize being the one cookbook which contains Rambly's chili recipe. When Jestro sends Moltor and Lavaria undercover to compete in the same contest, Eclair has the Nexo Knights seek a pepper so spicy, it explodes if mishandled. Meanwhile, Lance and Aaron hold their own contest to determine which of them is the most resistant to spicy peppers. Despite Jestro's efforts, the Nexo Knights succeed in getting the pepper they need and use it to win the cook off. But unfortunately, their chili ends up being so spicy, it burns the grand prize to oblivion once it was spilled.
The Nexo Knights send Clay on vacation to get some free time. Little do they know that Jestro and the Book of Monsters are on the trail of another evil spell book that is near where Clay is staying on his vacation. Clay soon discovers the staff of his vacation spot has been extremely rude, cruel and nasty to him (doing things such as serving him moldy food and forcing him to tip them for everything) courtesy of the Book of Cruelty. Meanwhile, the Nexo Knights are having fun. Problems arises because they started neglecting their daily duties and they rush to retrieve Clay. Despite his efforts, Clay is captured by Jestro and the Book of Cruelty is fed to the Book of Monsters which gives the Book of Monsters the ability to summon the Lava Monsters Crust Smasher and Flame Thrower. Fortunately, the other Knights arrive and save Clay and Jestro's forces retreat. To the misfortune of the Knights, Clay learns the importance of relaxing just as they had become more diligent.
During a dinner party that the Nexo Knights attend in the town of Cleanington, the Book of Monsters goes to Snottingham (the coldest place in all of Knigton) to consume the literally sickening Snotheiser which makes him and the Lava Monsters he summons very unhealthy. When the Nexo Knights rush to stop the attack on Snottingham, the Lava Monsters sneeze all over them causing them to wake up the next day with the Snottingham Flu (a version of the flu that causes bizarre effects on the victim in addition to usual flu symptoms) causing Clay to hallucinate, Macy to experience severe pins and needles, Aaron to become dizzy and uncoordinated, and Axl to be robbed of his hunger and appetite while the Book of Monsters makes a full recovery. Just before Jestro and the Book of Monsters are able to kidnap King and Queen Halbert, Merlok and Ava send a Nexo Power with an antivirus system added to it which cures the knights of their sickness and allows them to defeat Jestro and the Book of Monsters.
Inspired by Jestro's idol Jokes Knightley, Jestro and the Book of Monsters attend a gig in the village of Funnyton where they pose as comedians. The Nexo Knights also attend and to their surprise, they actually find Jestro hilarious, even attempting to convince Jestro to change his evil ways. However, Jokes Knightley kidnaps the Book of Monsters in order to steal Jestro's act, courtesy of the last evil spellbook called the Book of Betrayal. When the Nexo Knights find the Book of Monsters, it becomes a mad battle for the Book of Betrayal which eventually falls into the hands of Jestro. Not knowing what to do, Jestro runs into the woods and hands the Book of Betrayal to an unseen individual. Evil laughter is soon heard afterwards.
Recapping from the previous episode, the figure who Jestro gave the Book of Betrayal to is revealed to be Clay, who takes Jestro to the Fortrex. When the Book of Monsters attempts to gain the Book of Betrayal and Jestro, the Nexo Knights split up as a diversion until Clay is cornered. Now fully reformed, Jestro sacrifices himself so Clay can escape with the Book of Betrayal. The Nexo Knights then stage a rescue mission to save Jestro from the volcano lair where the Book of Monsters reveals that he is actually the dark necromancer Monstrox, whom Merlok turned into a book and divided his power among eleven other spellbooks. Monstrox adds that he needs Jestro as a host body to regain human form through a ritual in the "Dark Arena", waiting for the Book of Betrayal to corrupt the Nexo Knights. Unfortunately, Clay appeared to have succumbed to its power and begins to bring the Book of Betrayal to Monstrox.
Lance earns a leading role in 'The Golden Castle' and his friends play in the background. Macy declines but then eventually agrees to do so. When they're filming, Jestro summons a monster called Lavaria, a very sneaky monster; Jestro then takes a liking to her and orders her to find and give the Book of Monsters to eat the script of 'The Golden Castle' and hold its power, which they successfully do. But soon the knights find out, and they get the new Ultra Armour, giving them more powerful weapons, whilst the director is directing all the action.
Leaving the maze with the real Book of Deception, Jestro feeds several books of the same kind to the Book of Monsters to create the fake NEXO Knights. They raided and destroyed the city of Bork. The real knights come and all the citizens shooed them off for something they didn't do. Ava and Robin hear the news and are shocked by what they did. The monsters trick the citizens again by turning the Evil Mobile into the knights' Rolling Fortrex. When they tried to raid again, a Black Knight blocked their way and defeated them with ease. King Halbert then contacts the knights and said that if they don't come up with any evidence that they didn't destroy the cities, he'll be shutting them down. Several times the fake knights tried to attack a city, the Black Knight is there and defeats them in less than a few blows. The real knights came in afterwards in the real Fortrex and conquered over their fake selves. Soon, they come to the fact that Robin was in fact the Black Knight in disguise!
Jestro and the Book of Monsters travel to the place where the evil wizard Morstrux did his evil deeds (?) and find the Book of Destruction. Soon, Jestro brings out a whole lot of destructive monsters and split up in different directions. Macy scolded to her dad that she quits being a princess, but he disagrees and that she was born to be a princess (?). When the knights find out that monsters are attacking, Merlok 2.0 told them that they should go together, but they decided to split up; King Halbert hopes her daughter will come back. However, the knights get captured one by one, with no way to escape. When all the monsters were asleep, Aaron quietly escapes, being the only knight to save all his friends before Jestro attacks the castle of Knightonia.
Aaron must figure out a plan to save all his friends. Meanwhile, Macy has a flashback, saying that she wanted to become a knight, not being a princess all the time. Soon, she admitted to herself that she was not ready to be a knight, but was cut off by Aaron, who used his energy bow to cut the chain off of her, and retained her shield and mace back. The two then rescued Clay, who was chained on a target, getting spun around all the time. After him, they saved Axl by giving him a chain of sausages to climb up, but was unsuccessful the first time, and the second time, they succeeded. Finally, they rescue Lance, who was stuck in an animal farm, making a new friend, called Hamletta, a pig. Jestro, who was so close to victory, were stopped by the knights, and the rest sans Clay, got their NEXO Power, and the blue knight got one that was last done ages ago. When Clay got the new NEXO Power, one smash of his sword and all the monsters turned to purple dust, and he slammed his sword down again, and sent Jestro all the way out of the kingdom of Knightonia. Near the end, King Halbert apologizes to Macy, and he claims her an official NEXO Knight. Lance then gets Dennis back on one condition: Lance has to be the servant of Dennis for a week.
Party time in Knighton! The mood is down so the King decides to throw a party. In the meantime Jestro attacks the Fortrex, but he is confronted with the new Nexo Power "The Wedgie of Doom".
The Nexo Knights decide to use Clay to infiltrate the enemy camp but first Clay needs to learn how to be a good spy. Agent Shield, Jack Shield helps them with that.
Jestro and the Cloud of Monstrox builds a new stone army from giant building blocks to crush Knighton. The Nexo Knights try to convince Rockowski, a demolition expert, to help them.
Jorah Tightwad is digging for gold but finds the ancient Lord Krakenskull buried with his army. This stone army is exactly what the Cloud of Monstrox needs.
Clay Moorington the Nexo Knight has had enough of Monstrox, Jestro and their Forbidden Powers and decides to take the fight to them. But the Cloud of Monstrox is getting closer and closer to unleash his master plan.