During the outbreak of the rage virus in England, Don (Robert Carlyle) and his wife, Alice (Catherine McCormick) hide in a country house with four other people. Don and Alice are happy that their two children are on a school trip to Spain and thus away from the disaster.
They allow a young boy to enter the house but the infected have followed him and break in. The others are killed. Don abandons Alice to save himself.
Twenty eight weeks later, the infected have all starved to death. An American-led NATO force has secured parts of London and is now allowing the return of English citizens who had been abroad during the outbreak.
Snipers patrol the rooftops in case of another outbreak. Leaving the Green Zone (a secured area on the Isle of Dogs, a peninsula on the East End of London) is forbidden because troops are still cleaning up the corpses that have been rotting for six months.
Don and Alice's children, Tammy and Andy (Imogen Poots and Mackintosh Muggleton) arrive in the Green Zone. Don manages one of the apartment buildings in the zone. He tells Tammy and Andy that he tried to save their mother but there was no hope.
Tammy and Andy decide to sneak out into London and visit their old home to retrieve personal items. There, Andy discovers Alice in the attic, delirious but alive.
The NATO soldiers take Alice back to the Green Zone and subject her to an agressive scrub down. An Army medical officer, Scarlet (Rose Byrne), discovers an old bite wound on her and does a blood test.
Alice has the rage virus teeming in her blood but is immune to its effects. However she is a carrier who could infect others with her saliva or blood. The commanding officer, Stone (Idris Elba), orders Alice executed even though she could hold the secret to curing the virus.
Meanwhile, Don sneaks into where Alice is being kept. Tammy and Andy had bitterly accused him of abandoning her and he wants to explain himself. She tells him she loves him. They kiss tenderly and Don is infected with the virus.
Becoming an enraged madman, he crushes Alice's skull and escapes into the Green Zone, killing several soldiers and infecting others.
Realizing that Andy and Alice shared a genetic quirk that might make him immune to the virus's effects, Scarlet rescues Tammy and Andy. The Army has moved quickly to contain the infection, herding the residents of the Green Zone into safe rooms.
Andy is separated from Tammy and Scarlet and placed in a different room with other residents. However, Don has been hiding in this room and attacks the crowd, spreading the virus. Andy escapes through an air vent.