A highly contagious, rage-inducing virus is unleashed in Great Britain after a group of animal liberation activists free chimpanzees infected with the virus in Cambridge, spreading quickly among the populace and resulting in societal collapse. Twenty-eight days after the initial outbreak, bicycle courier Jim awakens from a coma in St Thomas’ Hospital in London, where he discovers the city deserted. Wandering through London and finding signs of catastrophe, he is attacked by humans infected by the virus and rescued by survivors Selena and Mark. The two inform Jim about the virus, which is believed to have spread worldwide. At Jim’s request, the group travels to his parents’ house in Deptford, where he learns they committed suicide. That night, Mark is bitten during an infected attack and Selena kills him before he turns.
After leaving the house, Jim and Selena encounter cab driver Frank and his daughter Hannah at Balfron Tower. Frank informs Jim and Selena of a military broadcast offering protection to survivors at a blockade in Manchester. With supplies dwindling, he asks them to accompany him and Hannah to the blockade. The two accept, and the group boards Frank’s cab to Manchester and become close with each other during the trip. When they reach the blockade, however, they find it seemingly deserted and Frank is infected when a drop of blood falls into his eye. The soldiers arrive soon afterwards and kill Frank.
The remaining survivors are brought to a fortified mansion under the command of Major Henry West. However, the safety promised by the soldiers turns out to be false when West reveals to Jim that the broadcast was intended to lure female survivors into sexual slavery to repopulate the world. Jim refuses to be complicit with the soldiers’ plan and they attempt to execute him, but Jim escapes. Jim tricks West into leaving the mansion and releases Private Mailer, an infected soldier kept chained for observation. This results in the deaths of West’s men. Jim, Selena, and Hannah attempt to leave in Frank’s cab, but find West sitting in the back seat. West shoots Jim. Hannah retaliates by putting the cab in reverse, allowing Mailer to grab West through the rear window and kill him, and the three survivors escape from the mansion.
Another 28 days later, Jim recovers at a remote cottage in Cumbria, while the infected are shown dying of starvation. As a Finnish fighter jet flies overhead, Jim, Selena, and Hannah unfurl a huge cloth banner spelling the word “HELLO.” The three survivors optimistically watch the jet as they hope that the pilot has spotted them.