London, four months after the attack. The survivors organize the Resistance. Bill continues his research on aliens and understands that their DNA is human but that their organs are degenerating. In France, a severely wounded dissident alien reaches the Observatory.
At the Observatory, Mokrani and Catherine clash over the fate to be reserved for Micah, whom Catherine considers to be a major source of information. Micah reveals to Catherine that she has to find Bill Ward. The latter, for his part, tries to find out more about Emily, still assailed by visions of Adina, the leader of the aliens urging him to hug Bill.
Without telling Emily or her parents anything, Bill decides with Zoe to test on Emily the biological weapon he hopes to be able to use against the aliens. Catherine, for her part, has had difficulty managing her grief since Mokrani's death. She now has only one thing on her mind: to find out who Bill Ward is and find him. Sophia is ready to help him, but Nathan hesitates.
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.