In London, electronic salesman Shaun has no direction in his life. He is disrespected by his colleagues, does not get along with his stepfather Philip, and is dumped by his girlfriend, Liz. Heartbroken, Shaun gets drunk with his close mate, Ed, at their favourite pub, the Winchester. At home, Shaun and Ed’s flatmate Pete complains of a bite wound from a mugger, and berates Shaun to get his life together.
By morning, a zombie apocalypse has overwhelmed London. Shaun and Ed are slow to notice until they encounter two zombies in their garden, whom they beat to death with a shovel and cricket bat. They devise a plan to rescue Liz and Shaun’s mother, Barbara, and then wait out the crisis in the Winchester. They escape in Pete’s car and pick up Barbara and Philip, who is bitten shortly after. They use Philip’s prized car to pick up Liz and her friends Dianne and David. Philip reconciles with Shaun before becoming a zombie.
The group abandon the vehicle and sneak through London neighbourhood, evading zombies by imitating them. They seek refuge inside the Winchester, where Shaun discovers that the Winchester rifle above the bar is functional. Barbara reveals to Liz a bite wound, and dies after giving Liz and Shaun’s relationship her approval. David attempts to shoot Barbara, but Shaun stops him, causing the group to argue. Shaun accuses David of being in love with Liz, which Dianne admits. Shaun, distraught, is forced to shoot Barbara when she reanimates.
Zombies break into the pub. David is disemboweled and devoured, and an enraged Dianne grabs David’s leg and rushes into the horde. The zombified Pete appears and bites Ed, and Shaun shoots Pete. Shaun, Liz, and a bitten Ed run toward the bar which Shaun sets ablaze and take refuge in the cellar. Realising they only have two bullets left, Shaun and Liz contemplate suicide while Ed elects to be devoured by zombies. Shaun discovers a hatch to escape into the street. Ed volunteers to stay with the rifle. The British Army arrives and take Shaun and Liz to safety.
Six months later, civilisation has returned to normal and surviving zombies are used as cheap labour and entertainment. Liz has moved in with Shaun, while Shaun keeps the zombie Ed tethered in his shed, where they play video games together.