It is the coming of the End Times: the Apocalypse is near, and Final Judgment will soon descend upon the human race. This comes as a bit of bad news to the angel Aziraphale (who was the angel of the Garden 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 God and Satan 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. Owing to a mix-up at birth, the real Anti-Christ is in fact Adam Young, a charismatic and slightly otherworldly eleven-year-old who, despite being the harbinger of the Apocalypse, has lived a perfectly normal life as the son of typical English parents and as a result has no idea of his true powers. As Adam blissfully and naively uses his powers, creating around him the world of Just William (because he thinks that is what an English child's life should be like), the race is on to find himthe Four Horsemen of the Apocalypse assemble and the incredibly accurate (yet so highly specific as to be useless) prophecies of Agnes Nutter, seventeenth-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.
Anathema teams up with Newton Pulsifer, the descendant of the man who initiated the burning of Agnes, to use the prophesies and find the Antichrist. Unfortunately, that is exactly what everyone else is trying to do, and time is running out.
Pratchett's work is simply amazing on so many different levels.
I even own them on ITV DVD. 
