It's a comedic take on the birth of the Antichrist. The boy who was born is monitored by a demon (Crowley) and an angel (Aziraphale). He is supposed to come into his evil powers at age 11, but when nothing happens on the boy's birthday, Crowley and Aziraphale discover that there was a mix up at the hospital and they've been watching the wrong boy who is ordinary and not the Antichrist. They have to work together to find the right boy. There are also a ton of funny subplots and characters, I'd really recommend it if you're into comedy books.