Ms.Shears dog, Wellington, was mysteriously murdered. Christopher was originally accused of this.
Hi sweetie! I just wanted to let you know why I left. Your father and I have been having arguments for a long time. I also did not believe I was a good parent to you. I had a short temper and did not know how to take care of you, with you being autistic and all. Mr. Shears just seemed to understand what I was going through at the time, so we moved to London. Your father refused to let me see you, so I am sending you these letters. I hope you understand. XOXO- Mother
After discussing the matter with Christopher, his father warns him to stop investigating the murder. What he doesn't know though, is that Christopher has already been documenting his investigation as a novel.
Christopher's father discovers his son's novel and begins yelling at him. He takes the notebook away and warns his son to never mention Mr.Shears' name in his house, though he does not tell him why.
As Christopher is looking around for his book, he finds it in his Father's room along with a collection of letters from his mom. Christopher was told by his father that his mother was dead, when in fact she had just left the family. This leads his father to also confess to killing Wellington after getting angry at Ms. Shears
Christopher decides that he can no longer live with his father, a murderer and a liar. Now that he knows his mother's new address, he decides that he will live with her. He runs away from home, and makes his way to the train station with only his special snacks and his father's pin number.
After finding his mother, Mr.Shears begins to argue with her more often. This leads her to take Christopher all the way back home to his father's house. His father wanted to repair his relationship with Christopher, so he adopted a dog in order to regain trust. The child then realizes that he wants to cooperate in order to have a semi-normal family again.