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.