Christopher gets home from school, and his father is still at work, so he goes about his business: he puts his school things on the kitchen table (including his detective book), makes himself a milkshake, and starts watching a video about life in the ocean.
His father comes home and finds the book on the kitchen table. This isn't going to end well.
He's really pissed, and Christopher tries to explain that technically he didn't disobey him, having tiptoed around the specific things his father said not to do.
Before he can finish, his father cuts him off: "Don't give me that bullocks, you little s***. You knew exactly what you were bloody doing. I've read the book, remember" (127.18). That's pretty harsh.
After some more yelling, he grabs Christopher roughly by the arm, something he's never done before. (Christopher says his mother used to hit him sometimes, but his father is more patient than she was.)
Christopher doesn't like being grabbed, so he starts hitting his father. And then he blacks out.
When he wakes up, he's sitting on the carpet: there's blood on his hand and the side of his head hurts.
Christopher's father is standing a few feet away. His shirt is torn, the detecting book is bent in half, and he's breathing heavily.
His father walks outside and puts the book into the garbage can, then comes back in and says he needs a drink. He gets himself a beer.