Main character, Christopher finds out his neighbour's dog(Wellington) is killed. He is sad because he is a dog lover, so he decides to find out who is the dog killer.Mrs. Shears, Wellington's owner doubts Christopher killed her dog since he is holding Wellington.
Christopher's father tells him that his mother has passed away by disease.
Christopher unexpectedly finds out his mother's (Judy Boone) letters. In the letter, Judy says she misses Christopher and wonders why Christopher has never give her a reply. Judy says she is now living in London with Mr. Shears and hope Christopher could visit one day. Christopher questions about the letters severely and his father says that he hid the letters for Christopher's good. Also, he says he killed Wellington by the heated up conversation with Ms. Shears. Christopher now looses the reliability toeards his father.
Christopher thinks it is not safe to stay at home sincce there is a murderer and decides to get on the train to visit Judy.Judy is surprised by Christopher and brings him home.
Judy decides to go back to live near Christopher's father's house for the good for Christopher. Christopher's father want Christopher to forgive him and tell him that he loves Christopher. He also gives Christopher a dog and wish him to own it with him.They slowly reconciles with his father.