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 unhappy 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 cuddle 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 cuddleous 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 'hugger 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.