The climax of the story is where all of the truth is revealed. When Christopher comes home from school one day, he decides to make a strawberry shake and relax while watching "Blue Planet". He carelessly leaves his book about Wellington's murder that he has been writing on the kitchen counter. When Father comes home, he sees it and reads through it. He asks if the conversation that Christopher had with Mrs. Alexander was true, and he responds with yes. Uncontrollable anger comes over him and he grabs Chris violently. Not liking being grabbed, Chris slaps his father and only remembers waking up on the floor, bleeding. His father cleans him up and apologizes as much as he could, and Chris decided to trust him again anyways. They go to the Zoo to try and mend the relationship. While Chris goes upstairs, he tries to find his notebook under his Father's bed. He finds his book, but along with that also finds a letter addressed to him from his Mother. Confused by this, he leaves it alone until the next night. The next night, he returns and finds 43 letters all addressed to him from his Mother. Every letter is postmarked after the date his Mother supposedly died in the hospital. Overwhelmed, Chris becomes dizzy, throws up, and faints.
The climax continues as his Father gives him a bath and puts him to bed. Before he sleeps, his Father tells him that he does not want to lie to him ever again, so he is telling the whole entire truth now. He tells Chris that his mother is alive, and that he was the one to kill Wellington. At that moment, Christopher loses his mind and begins to scream at Father. After the conflict calms down and Father is down stairs, Chris realizes that he cannot trust his Father. He thinks that if his Father had killed Wellington, that he could kill him too. He takes his special stash of food, his pet rat Toby, and his Father's money card and sleeps in the backyard, hidden.
After successfully hiding from his father all night, he proceeds to go to the train station. He is convinced that the only safe place to live is with his Mother in London. He goes through adventures on the train attempting to stay safe from cops that wish to return him to his Father, and keeping his pet rat Toby safe. Note that for someone with such a severe social disability like Christopher who is only 15, this is an enormous feat for him.
