Six months after Emily disappeared on an alien ship, the London survivors have set up a resistance base from which they plan to fight back; the aliens have since appeared and they look exactly like humans.

As Emily grows concerned by her recollections of time spent with the aliens, Bill is also perturbed by the similarities between Emily and the aliens; Zoe encourages a reluctant Bill to use these similarities to their advantage.

Bill secretly injects Emily with a biological weapon he hopes to use on the aliens; when she becomes sick, some of the survivors head out to find insulin in order to treat her; the resistance base is attacked and the survivors are forced to flee.

Catherine, Sophia and Nathan make their way to London, accompanied by Victor, a survivor encountered by chance. Along the way, the dangers multiply. Sacha's feelings towards Emily intensify and the young man's behavior becomes more and more disturbing. Feeling threatened, Bill is forced to flee.

Bill joins the military group formed by Ethan, Dominic and Michael. The three men hold a young alien hostage, Isla. Catherine, Sophia and Nathan continue to search for Bill. Adina, and the aliens also set out to find Bill. Chloe accuses the blow when she realizes that Sacha killed Ash. She suggests to her increasingly unstable son to run away with her.

At the university, Sophia watches the videos recorded by Dylan, a young man they found dead, while Catherine focuses on the algorithms he left behind and which decipher the behavior of robots. Thanks to Bill's intervention, Isla manages to flee and join her family.

Emily agrees to help Bill test her for the virus, but soon experiences serious side effects. Sacha, who has secretly followed the group to stay close to Emily, tries to remove her from the group. For their part, Catherine, Sophia and Nathan set out again in search of Bill Ward.

Catherine, Sophia, Zoe and the other survivors, finally reunited, prepare for the ultimate attack. But it is Bill who is the only one to hold the fate of humanity in his hands, provided he is helped by Isla.