London architect Martin Kennedy (Cillian Murphy) and his frustrated journalist wife Kate (Thandie Newton) frequently escape to the desolate Blackholme Island, located off the west coast of Scotland, for their holiday getaways. The island is home to Fairweather Cottage, run seasonally by its owner Doug (whose actor link is not provided), and is accessible solely via ferry. Having previously gone through a traumatic miscarriage, the couple's relationship has grown strained. In a bid to rekindle their bond, they embark on a return trip to the island.
A few nights into their stay, disaster strikes when the generator in the cottage explodes, injuring Martin’s arm and cutting off their electricity supply. They attempt to reach Doug via the CB radio, their only means of communication, but help is delayed. The tension mounts as Kate waits at the jetty for Doug, who never arrives. Soon after, a bloodied figure washes ashore—Private Jack Coleman (Jamie Bell)—who claims to be a soldier. He warns them of a devastating airborne illness called Argromoto Flu, codenamed R1N16. The disease originated in South America and has rapidly spread worldwide, proving entirely incurable and lethally contagious, causing victims to choke on blood.
As Jack explains that the military has lost control and civilians must remain confined, Martin makes the choice to trust him. Jack takes over, barricading doors and windows, but his behavior becomes increasingly erratic and unsettling. Doubts arise in Martin and Kate’s minds, raising suspicions that perhaps Jack is the threat rather than the disease. When Kate inquires about Jack’s marital status, he reveals his wife succumbed to R1N16, and his demeanor grows aggressive.
Desperate to escape, Martin and Kate attempt to leave the cottage; however, Jack uses force to restrain them, locking them in the bedroom at gunpoint. Martin cleverly sneaks out through a skylight and discovers the bodies of Doug and his wife by the pier, both victims of gun violence. Armed with Doug's hunting shotgun, Martin confronts Jack, gaining the upper hand. Yet, as Kate works to restrain Jack, Martin begins to cough up blood—signaling his infection with Argromoto Flu.
In a heartbreaking twist, Kate finds herself compelled to shoot Martin, ending his suffering before it spirals into a slow demise. Under duress, Jack confesses he had been experimented on at a military facility before being released, inadvertently becoming a carrier of the virus. He reveals he was responsible for his wife's death and had sought refuge on the island thinking he could quarantine himself. Moreover, Jack had manipulated the CB radio, ensuring Martin and Kate could not contact help.
After fixing the radio, the military broadcasts news of a supposed vaccine, but Jack warns Kate that the military's claims are false and that they would eliminate anyone who discovered their survival. Distraught over the potential loss of Martin, Kate lashes out and fatally shoots Jack. As she attempts to flee the island with Martin's lifeless body, a military helicopter appears overhead, and she meets a tragic end when a sniper’s bullet strikes her down.
