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.
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.
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.
When fractures within the Synth community start to appear, all hope of stability comes under threat, and both families are brought back to fight for the Green-Eyes' survival.
A grieving Max must face the consequences of his painful decision. Laura discovers she's not being told everything at the Dryden Commission, while Joe uncovers Karen's shocking secret.
As Leo struggles to come to terms with being human, Mattie takes him back to the Hawkins' home, where a surprise night out allows them to confront their feelings for each other. Laura is determined to uncover the truth about the too-secret Operation Basswood and tries to enlist Dr Neil Sommer's help, only for things to take an unexpected turn. Joe reluctantly decides to help Karen with Sam, and Niska receives a mysterious message.
Laura leads a tense Dryden Commission on a visit to the Railyard, but will Agnes' anger threaten its success? Mia comes face to face with Ed forcing her to confront her past, while an evening out for Karen, Sam and Joe turns to tragedy.
While Sam reels from the events of Waltringham and Joe struggles to help him through it, truths are revealed and dangerous plans put into action at the Railyard.
Stanley and Anatole's true colours put Laura and her family directly in harm's way. With Mia speaking at the commission, Laura must do everything in her power to stop an awful attack. Leo and Mattie return to the Elster house where Leo comes to understand a painful truth, but Mattie has a secret that could knock their fledging relationship off course. Niska continues her mysterious journey that takes her to Wales, where she must defy all her instincts to survive.
Max must confront a friend turned foe. Laura reels from her heartbreaking decision, but when Mattie reveals her secret, she is spurred back into action. Niska's journey takes an unexpected turn, while Leo races to be by Max's side, but walks into danger.
In a dramatic showdown, Mia, Max and the synths go into battle with humans; Laura, Niska and Mattie face monumental and life-changing decisions.
