London architect Martin Kennedy (Cillian Murphy) and his miserable journalist wife Kate (Thandie Newton) often visit a small, remote, uninhabited island off the west coast of Scotland called Blackholme Island for their holiday retreats. The only dwelling on the island, Fairweather Cottage, is seasonally operated by the owner Doug from the mainland and only reachable by ferry. After previously suffering a miscarriage, Kate's relationship with Martin has become tense, and in an effort to rekindle their marriage they decide on a return visit to the island. A few nights into their stay, the generator in the cottage explodes, injuring Martin's arm and leaving them without electricity. They use the CB radio, their only source of communication to the mainland, to call Doug, who ferries out to help them.Kate spends the next day waiting at the jetty for Doug to arrive, but to no avail. Later a mysterious man in green combat fatigues washes up on the island, bloodied and half-conscious. When he comes round, he identifies himself as a soldier, Private Jack Coleman (Jamie Bell). He tells Martin and Kate that there has been an outbreak of an airborne disease, Argromoto Flu, codenamed R1N16, which started in South America and has spread over the entire world in the time-span of a few weeks. The disease is incurable and highly contagious, attacking the respiratory system and causing the victims to choke on blood with a 100% fatality rate. Jack says that the military have lost control, and are now advising civilians to seal themselves up in their homes and not allow anybody access. With only static now coming from the CB radio, Martin decides to play it safe and help Jack, who aggressively takes command and boards up the door and windows.Over the next few days, Jack becomes increasingly strange and erratic, and his intimidating behavior begins to disturb the couple even further. They suspect that the virus may be a lie and that Jack is insane. When Kate asks him if he is married, he tells her his wife died of R1N16 and he becomes aggressive and threatening. Martin and Kate decide to leave the cottage and take their chances outside, but Jack refuses to let them, forcing them into the bedroom at gunpoint and locking them in. Martin sneaks outside through a skylight, finding the bodies of Doug and his wife at the pier, killed by gunshot. Using Doug's hunting shotgun, Martin returns to the cottage and gets the upper hand on Jack. However, just as Kate begins to tie Jack's hands behind his back, Martin suddenly begins coughing up blood; it appears the Argromoto Flu is very real and he is infected. Kate is forced to shoot her husband dead with the shotgun, to spare him a slow and agonizing death.Forced at gunpoint, Jack reveals to Kate that he was experimented on at a military compound and released without anyone having realized he was a carrier of R1N16. Jack had infected his wife, killing her, and fled to Blackholme Island to quarantine himself where the couple found him. Jack reveals that the CB radio worked all along, and he had just changed the settings so that Kate and Martin would not be able to reach anyone. Jack fixes the CB radio, and the military broadcast on it claims that the soldiers have a vaccine against the virus. Jack tells Kate that the military are lying; there is no vaccine, and the military would kill them both if they were discovered. Kate does not believe him and, angered by the possibility that Martin could have been saved, she shoots Jack dead. As she attempts to leave the island in the boat with Martin's body, a military helicopter flies over and she is killed by a sniper who shoots her in the head.