It is revealed in caption that a plague, called the 'Maze Virus', has previously swept through Europe, transforming its victims into a homicidal, zombie like state. Ireland was particularly ravaged by this virus, with much of the country evacuated or left in economic shambles. 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 defined categorically as the Cured and the Infected. The disease began five years earlier, and some of the Cured were in a feral state for nearly that long while Ireland was in a military-imposed national emergency. Many people in the general populace fear that keeping the Infected alive could easily lead to another outbreak, and there is a considerable amount of civil unrest by the Irish citizenry.Senan (Sam Keeley), a young Irish man in his twenties, is introduced as a Cured patient who is soon to be released to his sister-in-law's care. Senan is rather passive, whereas his Cured friend, Conor (Tom Vaughan-Lawlor), appears quite remorseless and blames society for their situation. Conor appears to have considerable, unspoken influence over Senan. Senan meets his 'Rehabilitation Officer', Sergeant Cantor (Stuart Graham), who clearly has little faith that the Cured can integrate back into society without further violence. Cantor acts like a parole officer and shows little compassion for Senan.Senan reunites with his sister-in-law, Abbie (Ellen Page), who has a young son, named Cillian (Oscar Nolan). Cillian quickly begins to bond with his uncle. Abbie soon inquires to Senan as to what became of her husband, Luke, Senan's brother who is still among the 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 his brother Luke, in the very flat that Abbie lives in. The Cured have a detailed memory of what they have done while infected; many of them suffer from post-traumatic stress.Conor, formerly a young lawyer with political aspirations, approaches his father seeking a job with a law firm. His father, also a lawyer, accuses Conor of killing Conor's mother and other atrocities while infected. Sergeant Cantor later confronts Conor and Senan and forcefully tells Conor to have no further contact with his family. Conor begins to form an underground political faction called the Cured Alliance, whose goal is ostensibly to regain many of the civil liberties that the Cured have lost. It is announced in various news broadcasts that the government is planning to euthanize the 25% (5,000 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 the Infected 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, but Senan initially appears uninterested, wanting only to settle into family life and forget the past. However, the public at large have become hostile towards the Cured living in their midst, and eventually Senan agrees to go on an arson mission with Conor to gain some political leverage. The mission was intended to be free of casualties, but a soldier is killed and Cantor suspects Senan and Conor. Cantor approaches Abbie at her flat and educates her on the nature of the people infected with the Maze Virus. He tells her that the infected are linked in some telepathic manner, and that they form into packs like predatory animals. He voices his suspicions that the predatory traits are not eliminated by the vaccine, and that the Cured are still very dangerous. When Abbie shows skepticism, Cantor shows her 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. Conor shows her a ghastly scar on his shoulder and tells her that her husband, Luke, did indeed try to fight back but was overcome by himself and Senan.Abbie confronts Senan, and he tearfully admits that he killed Luke after Conor infected him. She tells him to get out, and Senan finally realizes 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 after showing a rare emotional outburst, blaming Senan for betraying him. Conor asserts that Senan is a fool for believing that anyone other than other Cured people care anything about him now. Conor and the Cured Alliance set in motion a drastic plan to free the Infected from their cell-block. The Infected are freed from their captivity and they take to the streets, killing and infecting new victims and causing mass chaos. Irish Army troops are deployed to the streets in an attempt to quell the emergency. 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 and his fate is unknown. Senan, Abbie and Cillian return to their flat, but Cillian is bit by an infected person and begins showing immediate signs of infection. Senan, knowing he is immune, takes Cillian, and tells Abbie he will keep him safe until another vaccine is available.An unspecified time later, a news broadcast announces that order has been restored, although 8,000 new infected people are now in captivity. During the announcement Abbie is seen walking down the street alone. In the background promotional posters of Conor (as the voice of the Cured/Cured Alliance) can be seen; showing that he is not only alive but has obtained his goal of being a leader. It is publicized that some infected are now in hiding, as the government is again discussing euthanasia for the inevitable 25% who are Resistant. Watching the broadcast, Abbie bursts into tears, knowing that Senan is hiding somewhere with the infected Cillian, and that the government cannot be trusted but that the only hope for her son is another government vaccine.