What would happen if the Apocalypse was held and the Antichrist didn't show up?
That's the basic problem here. Aziraphale and Crowley are an angel of Heaven and a demon of Hell who have been assigned to Earth since the Fall, and have long since developed a spirit of cooperation so that they can both get their respective jobs done while remaining in the favor of their superiors. About eleven years ago, Crowley was given the proud honor of taking part in introducing the Antichrist to the world as a small baby by arranging for him to be swapped with the child of another human family in a hospital run by an order of satanic nuns (don't ask).
Only problem is, the nuns botched the job. Now the Antichrist is eleven years old, his supernatural powers are beginning to manifest themselves, the Four Motorcyclists of the Apocalypse (also don't ask) are ready to ride, the combined armies of Heaven and Hell are ready to march out and take care of issues once and for all, and if Crowley can't find the child before it all happens, things are liable to get ugly for him. Well, at least uglier.
Terry Pratchett wrote most of the words in this novel, and Neil Gaiman provided most of the basic story. Both of these authors are widely recognized and loved in their native United Kingdom, and if you're familiar with the Discworld novels or "the Sandman", you'd immediately understand why. The book is hilarious and clever without, somehow, being at all irreverent.
If that doesn't sell you, then go to your local public library, find a copy, and read the first three pages. That should do the trick.