The book opens seven minutes after midnight, when the narrator, Christopher John Francis Boone, finds Wellington, the poodle belonging to Mrs. Shears, his neighbor, dead on Mrs. Shears's lawn with a garden fork through its side. Christopher touches the dog’s muzzle and observes that it is still warm. He wonders who killed Wellington, and why.
Departing from his story (Christopher does this frequently throughout the novel), Christopher explains that he has difficulty determining people’s emotions from their facial expressions. But he can name each country in the world, their capitals, and every prime number up to 7,057. He recalls the first time he met Siobhan, eight years earlier. She drew faces on a piece of paper and asked him what emotions the faces expressed. Christopher could only identify the sad face, which represents how he felt when he found Wellington dead, and the happy face, which shows how he feels when he wanders the neighborhood at three or four in the morning. He could not identify the other emotions.
The story returns to Mrs. Shears’s lawn, where Christopher removes the garden fork and picks up Wellington. Mrs. Shears appears on her patio and yells at Christopher to get away from her dog. Mrs. Shears does not stop, even when he puts the dog down. So Christopher puts his hands over his ears and curls into a ball on the grass, trying to block out the sound.
Christopher reveals that we are reading his murder-mystery novel, written after Siobhan advised him to try writing a story he would want to read. Siobhan thought that the opening of the novel should grab people’s attention, which is one of the reasons that Christopher started it with Wellington's death. The other reason is that he could not start it any other way: this story actually happened to him, and he has trouble putting events in any order other than the order in which they occurred.
At the crime scene, two police officers arrive. Christopher initially finds their presence comforting, but he grows agitated when the policeman begins to ask him questions too quickly, seeming to implicate him in the murder. Christopher curls into a ball again, and he hits the police officer when the officer tries to lift him to his feet.
Christopher states that his book will not be funny. To be funny you have to tell jokes, and jokes often rely on the multiple meanings of words. The fact that one word can have multiple meanings confuses Christopher and makes him uncomfortable, so he will not put jokes in his book.