Fifteen-year old Christpher Boone knows all the countries of the world and their capital cities, and every prime number up to 7.507. He likes animals but has no understanding of human emotions. He doesn't like strange places or people he doesn't know. If people touch him, he will hit them.
When he finds the neighbor's dog Wellington murdered he decides to investigate the matter and to solve this mystery. It's a search that will ultimately upset his carefully constructed world.