The angel Aziraphale and the demon Crowley have spent a lot of time on earth, doing their respective duties and have got pretty comfortable here. So when the news reaches them that the Antichrist is about to be born and the apocalypse is drawing closer, they are not really happy about it. But Crowley sets things in motion, as they are supposed to be set in motion, though coincidence seems to have a hand in it as well. But it has all been predicted by Agnes Nutter, a witch who died a while back, but still has an heir in Anathema Device who keeps up the family tradition. While the Horsemen of the Apocalypse start to gather, it turns out that the boy everybody thought was the Antichrist was actually switched at birth and nobody really knows where the Antichrist really is.
Good Omens is exactly what you imagine a book written by Terry Pratchett and Neil Gaiman to be like. That is well-written, clever and a whole lot of fun.