In a parallel present day, where the latest must-have gadget for any busy family is a 'Synth' (a highly realistic humanoid servant that's transforming the way we live). In hopes of reducing their workload, a suburban family purchases a new Synth only to discover that sharing life with a Synth has quite far-reaching consequences.
The Hawkins, a loving but troubled family decide to buy a 'Synth' called Anita, but they start to suspect there's something unusual about her...
Laura is suspicious of Anita, and feels threatened by her presence. Hobb holds Fred prisoner in his facility, but while Hobb's superiors marvel at this unique specimen, Fred voices his fears about what this means to humanity. After a violent incident, Niska escapes her brothel prison, pursued by Karen and Pete. Leo's quest for information takes him to increasingly dangerous places. George's worst fears are realised when the health service give him a new Synth, Vera, who he immediately despises.
Laura finds herself at odds with her family when she tries to return Anita to the shop, but Laura is begrudgingly forced to keep her when Anita saves Toby from a near-fatal road accident. Joe suggests that Laura is imagining Anita's strange behaviour, but he can't help noticing how human Anita appears when he has to check her for damage. George seems to have submitted to Vera's joyless routine, but he is actually hatching a daring plan to escape with Odi. Pete and Karen investigate a murder at a brothel, but after Pete's hot temper earns him a suspension, he finds himself stuck at home with Simon.
Laura feels vindicated when Mattie tells her that she has seen Anita acting strangely. Laura insists that the family take Anita for testing and they're shocked to discover she's 14 years old. After meeting Leo through the Headcracker forum, Mattie thinks she might have an idea. Pete becomes stir crazy, stuck in the house with Simon.
News of the `Killer Synth' finds its way into the media, causing widespread panic, and placing Karen under even more pressure to track down the alluring fugitive Niska. However, it seems there may be an ulterior motive to her hunt for the murderous synth. Meanwhile, the Hawkins debate whether to keep Anita in light of their recent discovery that she is not quite as `new' as she seemed, and while Joe worries his domestic synth might inadvertently reveal his secret, Mattie's decision to place her trust in Leo could put an end to the family's arguments.
The fractured Hawkins family reaches its lowest ebb, with dad Joe in exile, and Mattie, Toby and Sophie tired of their parents' lies. As the situation reaches breaking point, Laura decides it is time to tell the truth, and her confessions has incredible consequences for household synth Anita. Meanwhile, Karen finally discovers `killer synth' Niska's whereabouts, and a joyful reunion for the runaways is cut tragically short when a brutal betrayal forces one of their number to make the ultimate sacrifice.
Rejected and alone, Karen sets out to George's house in search of Niska. Meanwhile, Pete is determined to find out who Karen really is, but his enquiries reveal a truth stranger than he could ever have imagined. The hunted synths gather at the Hawkins, grateful to Laura and family for providing refuge. As humans and synths live alongside each other in peace, could this be the harmonious future that Leo and co have longed for? But as Hobb gets closer, it may be too late for them to find out.
With the Synths now in captivity, Hobb attempts to extract Elster’s consciousness programming from their brains. There ends up being one final piece of the puzzle missing that nobody, including Leo could ever have imagined though. The devastated Hawkins family come to realise much Mia mean to them and their courage is put to the test as they take on Hobb and his forces in an effort to save the Synths from their imprisonment.
Niska is living as a fugitive in Berlin where an unexpected romantic encounter leads her to a huge decision with far reaching consequences. In the UK, Leo, Max and Mia are laying low in a coastal hideout while Mia works in a local cafe as 'Anita' to pay for their supplies and stay connected to the human world. Leo and Max come to the aid of Hester, a very special Synth, but when a mysterious organisation begins to pursue them, there's a tragic outcome. The Hawkins family are struggling to put past events behind them as they settle into a new home. In San Francisco, Athena Morrow, a fiercely-driven scientist researching AI is recruited by charismatic Milo Khoury, the entrepreneurial force behind tech company Qualia, for a top-secret project to create conscious Synths for the mass market.
Laura must decide whether to accept Niska's request, while Karen and Pete learn about a special Synth being sold for unprecedented amounts on the black market. Hester uses questionable means to gain information from her prisoner, and Leo faces a new mission. Mia takes a risk to help Ed but is her secret now in jeopardy?
Athena travels to the UK and her tragic secret is revealed. Niska's consciousness assessments begin. With more time at home Joe notices Sophie's increasingly eccentric behaviour. As Karen returns to work with Pete, cracks start appearing in their relationship. The Synths are forced to leave their hideout but Mia shocks them with a huge decision.
Mia and Ed get closer but their relationship is thrown into jeopardy when their secret is discovered. Toby gets closer to Renie but offends her when he breaks through her synthie mask. Things look bleak for Niska until Laura brings in a surprise witness. When Milo confronts Athena about her secret agenda, she persuades him her research could be mutually beneficial. Karen and Pete delve deeper into the mysterious 'seraphim' investigation. As Hester's influence over Leo becomes clear, cracks appear between him and Max.
When Mia risks falling into the hands of a mysterious organisation, she is forced to take drastic action. Odi struggles to find meaning, while Sophie is awestruck by Renie's 'Synthie' lifestyle. As Pete recovers from his assault, Karen makes an admission that rocks their relationship. Hester and Leo stake out the Silo and track down the sniper who killed Ten. Hester agrees to deal with her peacefully, but can she be trusted? Mattie leaves home to reconnect with Leo.
When Mattie reveals that the Synths are planning to break into the Silo, Laura is determined to find Niska and ask for her help. Mattie asks Max to help save Leo from Hester's influence. Toby receives a disturbing message from Renie. Karen seeks out Athena's expertise and makes a life-altering decision. Mia and Hester break into the Silo, but things don't go according to plan.
The acclaimed drama concludes. In the aftermath of the Silo tragedy Karen feels she and Sam have no future and takes matters into her own hands. Niska and Astrid prepare to return to Berlin, but can Niska really walk away from her family? Joe leaves Laura shell-shocked with a frank admission. Hester seeks revenge and will stop at nothing to hurt Leo.
Athena challenges Milo's ethics and is shocked when he reveals his real plan. Mia's surprise return has ramifications for the Hawkins family. Mattie finds Leo but can she trust him with her secret? Max resolves to begin a new community of Synths; when he learns how far Leo is prepared to go to complete his mission, he takes matters into his own hands.