A couple buy a synthetic human, or 'synth' to help around the house, but the new arrival threatens to expose their secrets. A retired engineer's paternal relationship with his own synth is threatened by a malfunction, an android and his owner go on the run, and a detective grows suspicious of his wife's close bond with her synth.

Laura becomes increasingly suspicious, feeling threatened by the presence of Synth Anita, the 'so-called' perfect mother figure in her home. 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 Anita has saved Toby's life in a road accident and is commended by most of the family Laura is dissuaded from sending her back and even begins to warm to her, though she is baffled by the synth's unusually high intelligence and suspects illegal modification, which Anita denies. Meanwhile, to stop Odi from being taken away and recycled, George lets him loose in the woods. Hobb, still trying to find out from Fred the whereabouts of the other missing synths, arrives at the brothel where Niska has killed a punter and he is recognized as being an authority figure.

Laura begins to believe that Anita may be able to develop human traits after meeting a woman who claims her synth is also sentient though whilst she is out Joe has sex with Anita. Leo meets up with Matty but she refuses to admit knowing Anita. However Leo and Alex call on George, who worked on the original synth project with Leo's father and reveals that Leo's father imbued several synths with an alternative programme to give them consciousness. Leo then gets a call from Niska, who has infiltrated an undercover fight club where synths are systematically smashed up but where she has attacked the human punters before escaping. Meanwhile Laura and Joe are amazed to find that Anita is not the brand new model they were led to believe but a fourteen year old synth who has been reprogrammed.

As news of the 'killer Synth' spreads, Karen comes under increased pressure to find Niska. The Hawkins family debate whether they should keep Anita. Joe worries that his secret will be discovered and Mattie puts her trust in Leo.

With the Hawkins family at their lowest ebb, Laura makes a confession that will have serious implications for Anita and leaves Mattie wondering if she can ever forgive her. Meanwhile, Karen discovers the whereabouts of Niska.