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.
George's worst fears are realised when he is assigned a new Synth whom he cannot stand. Laura feels threatened by Anita's presence in her home, and Fred is held captive in Hobb's facility, which leads him to fear for the future of the human race. Karen and Pete have to hunt down Nisha after she escapes from the brothel, while Leo's search for information leads him down a dangerous path.
Laura is begrudgingly forced to keep Anita when she saves Toby from a road accident. George is planning an escape with Odi and Pete's hot temper earns him a suspension from work.
Laura insists the family take Anita for testing when Mattie tells her that she's seen Anita acting strangely, while Hobb is on Niska's trail
Killer Synth is the breaking news story, and public panic is growing. Karen is hot on Niska's tail, but are her motives for finding her more personal than professional?
Things are at the lowest possible ebb for the fractured Hawkins family. With Joe in exile and the kids tired of their parents' lies, Laura decides it's time to tell the truth.
Rejected and alone, Karen sets out to George's house to find Niska. Meanwhile Pete is determined to find out who Karen really is. The hunted Synths take refuge at the Hawkins.
With the Synths in captivity, the devastated Hawkins' realise how much Mia and her family mean to them and are determined to help them