A gifted 15 year old boy with autism named Christopher Boone lives in the small town of Swindon. Whilst he is preparing for a very important test that would prove his superiority over the others of his school, his preparation is abruptly interrupted by the discovery of Mrs. Shears, his neighbor's, murdered dog (Wellington) in their garden.
He decides to start his very own investigation to find the dog’s murderer. His father finds out about this investigation of his, and demands Christopher to stop sticking his nose into other people’s business. Despite his father’s specific orders, Christopher continues his inspection.
During Christopher’s investigation on Wellington's murder, he learnt that his mother and Mr. Shears had an affair, and so did his father and Mrs. Shears.
He finds out through some handwritten letters from his mother about his father's surreptitiously hidden lie about his mother's death. This shocking discovery puts a sudden halt in his investigation. Well, more of an end to the investigation, because his father reveals right after that he had murdered the dog in a bout of anger.
After recovering from this overwhelming barrage of new information and quite the bombshell of revelation, Christopher travels to London to join his mother by taking the subway. He arrives to find his mother living with Mrs. Shears' husband.
His plan to stay at his mother’s place is foiled when both his mother and him are forcefully kicked out of the house. He returns to Swindon and slowly starts to rebuild his relationship with his father. He finally takes his A level math's exam and successfully passes it with high grades.