Gary King is a forty-year-old alcoholic, who still retains the mindset and immaturity of his teenage years, whereas his four childhood friends, Peter, Oliver, Steven, and Andy, have all matured; and each of the four has become estranged from him. One day, Gary, in an attempt to recapture the days of his youth, tracks down his friends to complete the “Golden Mile”, a pub crawl encompassing the 12 pubs of their hometown of Newton Haven, the last of them being the World’s End. The group attempted the crawl as teenagers, but failed to reach the final three pubs. Andy, now a teetotaller due to a drunk-driving accident in which he and Gary were involved years ago, agrees to join them despite being uncomfortable with the idea.
The crawl begins, and in the second pub, the group are joined by Oliver’s sister Sam, whose affections Gary and Steven fought over in school. Throughout the beginning of the crawl, the town residents appear eerily stoic and none seems to recognize the group. In the toilets of the fourth pub, Gary picks a fight with a teenager and knocks his head off, exposing him as an android. Gary’s friends join him and fight the remaining members of the teenager’s gang, all revealed to be androids themselves. The group realize that the entire town’s population has been replaced with androids, which the group later refer to as “blanks”. In shock, Andy abandons his teetotal lifestyle.
Despite this realization, Gary urges the others to continue the pub crawl so as not to arouse suspicion. As they continue they discover that aliens intend to build a prosperous, galactic conglomerate, with humanity beside them; but any humans refusing to join this are to be replaced with blank versions of the original human hosts.
Oliver is revealed to have been replaced, and Peter is captured. Despite this, Gary is still determined to finish the pub crawl, for he believes it is all he has left. Andy is forced to abandon Steven when the blanks attack, and chases after Gary to the final pub. At the World’s End, Andy confronts Gary and the two fight. In the midst of the scuffle, Andy reveals his marriage is in trouble; and Gary’s sleeves are torn off, revealing bandages applied after a recent suicide attempt. Andy tries to stop Gary from drawing his final pint, but Gary states he clings to his goal of completing the Mile due to his sense that he has not achieved, and cannot achieve, anything else in life.
Once Gary attempts to pour his pint, the pull of the lever reveals a hidden chamber. A disembodied alien entity, known as the Network, tells them that the blank invasion has been responsible for the technological advances in telecommunication over recent decades as part of a first step to joining a galactic community. It offers Gary eternal youth if he agrees to become a blank, but Gary refuses. Along with Andy and Steven, who had survived the blanks attack, Gary gets into a foul-mouthed debate with the Network, calling out the tyranny in the latter’s plan and demanding that humanity should be left to its own devices. Eventually, the Network, exasperated, abandons the invasion; the departure triggers a nuclear pulse across the town, but Sam arrives in time to drive the group out of Newton Haven as it self-destructs.
Some time later, the destruction of Newton Haven has triggered a worldwide electromagnetic pulse that destroyed all electrical power on Earth, sending humanity back to the Dark Ages. The remaining blanks have reactivated, achieving independence without the presence of the Network, but are mistrusted by surviving humans. Andy’s marriage has recovered, Steven is in a relationship with Sam, and the blank versions of Peter and Oliver have returned to a semblance of their former lives. In the ruins of Newton Haven, the now-sober Gary enters a pub with the blank versions of his younger friends and orders water. When the bartender refuses to serve blanks, Gary leads his new friends into a brawl.