One night, Christopher Boone finds his neighbor's dog dead in her front yard, with a pitchfork sticking out of it. Eek – we're off to an interesting start, that's for sure. Anyway, Christopher wonders who killed it, and decides to write a book in which he tries to figure it out, like a murder mystery novel.
Christopher has a disability – unspecified in the book, but which has been compared with an autism spectrum disorder called Asperger syndrome – that makes it difficult for him to understand social norms like body language and other forms of human interaction. He is, however, tremendously good at math and more logic-based skills (like writing a crazily-detailed daily schedule, or drawing intricate maps of places he's only visited once).
Christopher's neighbor, Mrs. Shears, finds him with her (now-dead) dog, calls the police, and Christopher has to spend a few hours in a jail cell. Eventually, his father comes to get him, and tells Christopher to not investigate the dog's death any further. So, in response, Christopher thinks of all kinds of ways to interpret his father's demand as specifically as possible… so he can still do all of his detective work while somehow not disobeying him.
He starts asking around the neighborhood to see if anyone knows anything about the dog's death. He decides that since Mr. Shears left his wife two years ago, perhaps he hates her, and killed her dog to make her sad. (Seems like a stretch, but you never know.) When Christopher's father finds out he's been asking people about the dog, he makes him promise he'll stop. Again. Christopher promises.
So, of course, Christopher continues talking to one of his neighbors, who tells him that his (Christopher's) mother and Mr. Shears were having an affair before he left Mrs. Shears. That's bad news. But Christopher tells her that his mother died two years ago, of a heart attack.
Christopher's father finds the detective book Christopher has been writing, in which he's recorded everything that has happened so far. He's really mad about it, and takes the book away. A few days later, Christopher searches the house for the book, and finds it hidden in his father's bedroom. But here's the kicker: he also finds a big stack of letters addressed to him, from his mother. He reads a few of them, and discovers that – wait for it – she's actually still alive! His father had been lying to him this whole time.
His father apologizes for lying, and also admits that he was the one who killed Mrs. Shears' dog. As it turns out, he has feelings for Mrs. Shears, and was mad that she didn't want to be with him. Whoa.
Christopher decides that living with his father is no longer such a great (or safe) idea – he is a dog-killer after all – and thinks it's best to move to London and live with his mother. Problem is, he's never gone anywhere by himself before, and has difficulty being in busy places and/or around large groups of people. The journey is, as we might then expect, incredibly challenging. First, after he runs away, his father enlists the police to try to find him. He manages to escape anyway, but then he's totally overwhelmed, being on his own like this. He repeatedly vomits and passes out and just feels horribly sick.
Hours and hours later, he arrives at his mother's apartment in London. She's living with none other than Mr. Shears. Christopher tells her that his father said she was dead, and she's horrified to learn this. When Christopher's father comes to find him, she demands he leave and insists that poor Christopher can live with her. But Christopher is afraid of Mr. Shears, and is quite eager to go back home to take an important exam that will help him get into university.
After about a week, he and his mom go back home, and Christopher takes the exam (even though he can't think straight, after not eating or sleeping for days on end). His mother gets a job and a not-so-nice apartment, which Christopher hates. Meanwhile, his father tries very hard to earn back his trust. He buys him a puppy (that's some brownie points right there), and Christopher begins spending some time at his house again.
He receives his exam results, and finds out that he got the best possible score. Having successfully traveled to London on his own, and solved the mystery of who killed the dog, he's sure he can do anything. We agree.