It is the coming of the End Times: the Apocalypse is near, and Final Judgement will soon descend upon the human species. This comes as a bit of bad news to the angel Aziraphale (who was the guardian of the Eastern Gate of Eden) and the demon Crowley (who, when he was originally named Crawly, was the serpent who tempted Eve to eat the apple), respectively the representatives of Heaven and Hell on Earth, as they have become used to living their cozy, comfortable lives and have, in a perverse way, taken a liking to humanity. As such, since they are good friends (despite ostensibly representing the polar opposites of Good and Evil), they decide to work together and keep an eye on the Antichrist, destined to be the son of a prominent American diplomat stationed in Britain, and thus ensure he grows up in a way that means he can never decide between Good and Evil, thereby postponing the end of the world.
Unfortunately, Warlock, the child everyone thinks is the Anti-Christ is, in fact, a perfectly normal eleven-year-old boy. Due to mishandling of several infants in the hospital, the real Anti-Christ is Adam Young, a charismatic and slightly otherworldly eleven-year-old living in Lower Tadfield, Oxfordshire. Despite being the harbinger of the Apocalypse, he has lived a perfectly normal life as the son of typical English parents and as a result has no idea of his true powers. He has three close friends, Pepper, Wensleydale and Brian, who collectively forms a gang called the Them.
As the end of the world nears, Adam blissfully and naively uses his powers, changing the world to fit things he reads in a conspiracy theory magazine, such as raising the lost continent of Atlantis (causing an international crisis) and causing Little Green Men to land on earth and deliver a message of goodwill and peace (though they have no idea why). In the meantime, the Four Horsemen of the Apocalypse assemble in the form of bikers (War is a female war correspondent; Death resembles his Discworld counterpart; Famine is a dietician and fast food tycoon; and Pestilence, having retired after the discovery of Penicillin, has been replaced by Pollution), and the incredibly accurate (yet so highly specific as to be useless) prophecies of Agnes Nutter, 17th-century prophetess, are rapidly coming true.
Agnes Nutter was a witch in the 17th century and the only truly accurate prophet to have ever lived. She wrote a book called The Nice and Accurate Prophecies of Agnes Nutter, Witch, a collection of prophecies that did not sell very well because they were unspectacular, cryptic and, ironically enough, all true. She, in fact, decided to publish it only so that she could receive a free author's copy. There is only one copy of the book left, which belongs to her descendant Anathema Device. Agnes was burned at the stake by a mob (because that is what mobs did at that time); however, because she had foreseen her fiery end ("Ye're tardy; I should have been aflame ten minutes since") and had packed 80 pounds of gunpowder and 40 pounds of roofing nails into her petticoats, everyone who participated in the burning was killed instantly.
As the world descends into chaos, Adam attempts to split up the world between his gang. After realizing that by embracing absolute power, he will not be able to continue to grow up as a child in Lower Tadfield, Adam decides to stop the apocalypse.
Anathema, Newton Pulsifer (the last member of the Witchfinder Army), Adam and his gang, Aziraphale and Crowley gather at a military base near Lower Tadfield to stop the Horsemen causing a nuclear war and ending the world. Adam's friends captures War, Pollution, and Famine. Just as Adam's father, the devil, seems to come and force the end of the world, Adam twists everything so his human father shows up instead, and everything is restored.