Christopher Boon, a mathematically gifted boy of fifteen with Asperger's syndrome, sees a dog lying on the grass in front of his neighbor’s house with a fork sticking out of it. The dog is dead. He goes over to hug the dog and soon the neighbor, Mrs Shears, emerges from her house, screaming. She thinks Christopher has killed the dog.
The police arrive and Christopher is taken to the station and, shortly after, his father arrives to testify for Christopher and take him home. The strange death of Wellington prompts Christopher to search for his killer. As a school assignment, Christopher’s class is told to write a story. Inspired by this, he decides to write a detective story, the very one we are reading: The Curious Incident of the Dog in the Night-time.
Taking his inspiration from the Sherlock Holmes stories, Christopher knocks on the doors of various neighbors to try to find out what happened to Wellington. His father is furious when he finds out what Christopher is doing, and forbids him from continuing his ‘detecting’. Not understanding the logic behind his father’s aggression or his insistence that he stop, Christopher continues his search and talks to one of his neighbors, Mrs Alexander. Christopher asks her about Mrs Shears and asks, since Mr Shears left his wife, if she thought he had something to do with the murder. Mrs Alexander reveals that Mr Shears and Christopher’s mother were having an affair, before Christopher’s mother died two years before.
Not long after, Christopher’s father discovers his book, screams at Christopher, grabs him and throws his book away. While at work, Christopher looks for his book among his father’s belongings and finds letters addressed to him, from his mother at an address in London, written a year and a half after she died. Once Christopher realizes his father lied to him, he feels sick and lies in bed. His father finds him and realizes what has happened and apologizes, telling Christopher he was only trying to protect him and that he can trust him. He then tells Christopher that it was he who killed Wellington because, after a relationship with Mrs Shears when Christopher’s mother had left, he had an argument with her and the dog came after him and all his feelings just came bubbling up.
Terrified by the news of his father as a murderer, Christopher slips out of the house with his pet rat Toby and a swiss army knife while his father is asleep. He starts his journey to find his mother in London. The police have been informed that Christopher is running away and they try to catch him. At one point during the journey he loses Toby and follows him along the tube tracks, getting back onto the platform just before the train arrives.
Christopher finally arrives at his mother’s and tells her what has happened and that he thought she was dead. He goes to sleep and wakes up to hear his father shouting. His father enters Christopher’s room and tries to apologize. Christopher won’t talk to his father and insists he lives with his mother, thus ending her relationship with Mr Shears. He takes his A-level in maths and finally allows his father back into his life. His father suggests that they work on a project together where he proves to Christopher that he can be trusted and, to show how serious he is, buys a puppy that they can look after together. They begin spending more time together. Christopher plans to take further maths A-level a year later, and then physics A-level, and then he plans to get a first class honors degree and become a scientist – he knows this is possible because he solved a mystery, went to London alone and wrote a book.