When astrophysicists detect a transmission from another galaxy, it is definitive proof of intelligent extra-terrestrial life. The world's population waits for further contact with baited breath. They do not have to wait long. Within days, mankind is all but wiped out; just pockets of humanity are left in an eerily deserted world.

After the devastating alien attack, disparate pockets of survivors emerge into a shattered world. They quickly realise they must put their differences aside if they are to stand any chance of survival. For they are not alone. Something sinister lurks in the shadows and whatever it is, it won't consider its victory complete until it has killed every last human. But who are these attackers and why are they hell-bent on destruction?

As the dust settles on their brave new world, the survivors are forced to face some cold truths. Catherine is stunned to discover the part she played in bringing about the alien attack. Helen becomes fixated on revenge. And Emily momentarily regains her sight. But is the miracle cause for celebration, or concern?

The survivors make some disarming discoveries not only about the aliens but also about themselves. Mokrani goes on the offensive, taking the fight to the aliens. Ash is forced to make the hardest decision of his life. And Chloe’s past catches up with her, threatening to rip apart the delicate semblance of normality she is so desperately clinging on to.

Cracks start to show as the survivors find different ways to cope. Jonathan’s increasingly perturbed by Sacha’s strange behaviour. Emily proves to be an exciting conundrum for Bill. And Mokrani is horrified as his men delight in the opportunity to exact revenge.

The alien’s connection to certain survivors raises difficult questions. Bill makes a startling discovery. Sacha’s behaviour is increasingly erratic. And Tom’s birthday proves cause for celebration. But at what cost?

Tragedy strikes, altering the course of the future forever. Catherine’s priorities change with the arrival of Sophia. Things come to a head between Jonathan and Sacha. And Emily’s so happy she can see again that she’s stopped asking why, even if others haven’t.

The survivors attempt to turn the tables on their aggressors once and for all. Mokrani and Catherine prepare for the fight of their lives. Emily seeks answers, at any cost. And Sacha’s visions continue, giving a confounding glimpse of the future.

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.