Christopher is 15 and has aspbergers syndrome, he lives with his father in Swindon. His mother is dead.
Christopher finds his next door neighbour's, Mrs Shears, dog dead. A policeman turns up and Christopher hits him when he gets touched by him.
Siobhan, one of Christopher's school teachers, tells Christopher he should write a book about the murder so he sets off trying to find out who killed the dog and writing it in a book.
His father tells Christopher he cannot write in the book anymore and that he must stop investigating. Father takes the book and hides it.
Christopher finds letters from his mum while trying to find his book and realises she is still alive. This makes him very sick and when his father is helping clean him up he admits to Christopher that he killed the dog!
Christopher is scared living with his father knowing that he is the murderer, so he gets on a train to try and find his mum who he now knows lives in London.