It's the story of a angel (Aziraphale) and a demon (Crowley/Rampa in french) that work together to find the Antechrist, son of Satan. But the protagonists are so funny ! The story is hilarious.
Terry Pratchett and Nail Gaiman did a very good job. I am so disapointed Terry Gilliam wasn't able to get the money to make a film based on it.