A plague called the Maze Virus has previously swept through Europe, transforming its victims into a homicidal, zombie like state. Ireland was particularly badly affected. A cure was later discovered, and 75% of those infected who could be captured have been treated and cured of the virus. The remaining 25% have proven resistant to the cure. The two groups are known as the Cured and the Infected. The Infected are held in captivity, despite public fears that keeping them alive could lead to another outbreak.
Senan and Conor are Cured patients who have been interned together. Their ‘Rehabilitation Officer’, Sergeant Cantor, has little faith that the Cured can integrate back into society without further violence, and treats them with hostility. Senan reunites with his sister-in-law, Abbie, who has a young son, Cillian. Abbie asks Senan what became of her husband, Luke, Senan’s brother, who is still missing. Senan claims ignorance of his fate, though it is revealed in one of Senan’s many flashbacks that Conor infected Senan, who then killed Luke, in the flat that Abbie still lives in. The Cured have a detailed memory of what they have done while infected; many of them suffer from posttraumatic stress disorder. Conor, formerly a lawyer with political aspirations, cannot find work and is shunned by his family for his acts while infected. He forms an underground movement called the Cured Alliance, with the aim of regaining the civil liberties that the Cured have lost.
The government is planning to euthanise the 25% (5000 patients) who are infected, despite the assertions of a prominent virologist that a better vaccine for the Infected is nearly developed. Senan takes a job at the hospital where the Infected are housed, and observes that they show no interest in attacking the Cured. This is a source of political tension in the community, as the never-infected suspect aloud that the Cured would love to see another outbreak of the Maze Virus, as they would be largely immune to its effects.
Conor attempts to recruit Senan into the Cured Alliance. Senan initially refuses, but after experiencing the hostility of society towards the Cured, he takes part in an arson mission for the group. Conor claims that the targets are uninhabited houses, but a soldier is killed and Cantor suspects Senan and Conor. Cantor approaches Abbie at her flat and tells her that the infected are linked in some telepathic manner, and that they form into packs like predatory animals. He suggests that the predatory traits are not eliminated by the vaccine, and that the Cured are still dangerous. He shows Abbie a photograph of Conor and Senan roaming together when they were infected. Conor appeared to be leading Senan in murderous acts in the photo. Conor later approaches Abbie, and tacitly admits that he and Senan killed her husband while they were infected during the early days of the outbreak.
Abbie confronts Senan and throws him out. Senan realises that Conor has no remorse for what he has done in the past, and what he is still doing. Senan assists Cantor in a sting operation to arrest Conor after he confesses aloud to the arson; however, Conor violently murders Cantor and flees. The Cured Alliance initiate a plan to free the Infected from their confinement. The Infected take to the streets, killing and infecting new victims and causing mass chaos. Irish Army troops are deployed. Senan finds Abbie, who is frantically trying to retrieve Cillian from his school. Senan rescues Cillian but is confronted by Conor, who beats him savagely. A soldier shoots Conor but he escapes. Senan, Abbie and Cillian return to their flat, but Cillian is bitten and infected. Senan, knowing he is immune, takes Cillian, and tells Abbie he will keep him safe until another vaccine is available.
Some time later, a news broadcast announces that order has been restored, although 8,000 new infected people are now in captivity. During the announcement, promotional posters of Conor (as the voice of the Cured Alliance) can be seen. It is publicised that some infected are now in hiding, as the government is again discussing euthanasia for the resistant 25%. Senan goes into hiding with the infected Cillian.