The film was a box office success. Having been made on an estimated budget of only $8,000,000, it went on to earn more than $82,000,000 at movie theaters.
28 Days Later was rated R by the Motion Picture Association of America for strong violence, gore, bad language and nudity.
British animal liberation activists break into a laboratory in Cambridge. While trying to free some chimpanzees being used for medical research, they are interrupted by a scientist. Despite his desperate warnings that the chimps are infected with a virus called "Rage", which he claims is highly contagious and only takes one bite to spread, the activists ignore him and open the cages anyway and release the chimpanzees.
28 days later, a bicycle courier named Jim awakens from a coma in St Thomas' Hospital in London. He finds the hospital deserted and seemingly empty. He ventures out to discover the city completely deserted with signs of catastrophe everywhere. Jim then wanders into a seemingly abandoned church, only to alert a small group of Rage-Infected people who were hiding there.
Afterwards, they rush him to their hideout in the London Underground. There, they reveal that while Jim was comatose from his accident, an unknown virus spread uncontrollably among the populace, turning most people into rabid, psychotic "Infected", overwhelming the government and security services, resulting in societal collapse. They also explain that infection has been reported in Paris and New York. The next morning, Selena and Mark accompany Jim to his parents' house where he discovers that they have committed suicide. That night, two of the Infected see a candle Jim lights in the kitchen and attack. Mark is badly cut and covered in Infected blood; Selena quickly kills him, later explaining to Jim that the Rage virus overwhelms its victims in no more than thirty seconds. This necessitates the immediate killing of people who may have been infected. She also assures him that, should he get infected, she would kill him "in a heartbeat". As he succumbs, he is killed by the arriving soldiers, who then take the remaining group to a fortified mansion under the command of Major Henry West.
After settling in the mansion, West promises the three protection from the Infected. Jim attempts to escape with Selena and Hannah, but is captured by the soldiers, along with a Sergeant Farrell, who disagrees with the major's plan and tried to stop the other soldiers from imprisoning the group. During their imprisonment, Farrell theorizes that there is no worldwide pandemic, and that only Great Britain has been quarantined, which is proved when a NATO plane scouts the land.
Selena and Hannah rush Jim to a deserted hospital, where Selena performs life-saving emergency procedures, though Jim falls into the coma again. They take his body to the cab and leave Manchester.
28 days later, Jim is shown waking up in recovery again, this time at a remote cottage. Downstairs, he finds Selena sewing large swaths of fabric when Hannah appears.