At school, Siobhan suggests turning Christopher’s book into a school play, but he isn’t interested.
Later, Christopher runs away from home. He briefly stops at Mrs Alexander’s place to ask if she can care for his pet rat, but ends up taking the rat with him when she mentions contacting his father.
Christopher then encounters a passerby, and after a somewhat jumbled conversation about directions to the train station, a police officer approaches him at the station. The officer notices Christopher has his father’s credit card.
Despite the officer’s attempts to remove Christopher from the train after contacting Ed and learning that Christopher has run away, Christopher manages to board the train to London. The officer tries to stop him, but the train departs before he can get Christopher off, leaving them both on the moving train.
Christopher explains the way his thoughts are different from most people’s, in that he observes and counts everything with great detail. He demonstrates this by listing everything he sees out the window.
The police officer notices that Christopher is wetting himself, and directs him to the toilet. After exiting the toilet, Christopher evades the officer by concealing himself in the luggage area and goes unnoticed. He gets off the train in London, and attentively reads every word on the signs and advertisements around him. When a station guard approaches him, Christopher brandishes his Swiss army knife defensively.
With help from someone at the information desk, he makes his way to the tube platform. Seeing that Toby has escaped his cage and run on the tracks, Christopher jumps down to get him, much to the alarm of the other passengers. He retrieves Toby and narrowly climbs back up to the platform before a tube approaches.
After arriving at Willesden Junction by tube, Christopher exits the station and purchases an A-Z map to locate his mother’s house.
Upon reaching the house, he is greeted at the doorstep by his mother, Judy, and Roger Shears.Judy is overjoyed to see him and welcomes him inside the flat. When his mother asks why he didn’t respond to her letters, Christopher reveals that Ed had told him she was dead, leaving her infuriated.
After receiving a report that Christopher has gone missing, a police officer shows up at the flat. Fearing he might be taken back to his father, Christopher locks himself in a room. However, his mother assures him that he can stay with her, leading the officer to leave.
Soon, Ed shows up and has a heated argument with Judy about where Christopher should live. Although Ed pleads for Christopher’s forgiveness, Christopher remains silent. Eventually, the police officer comes back to escort Ed away.
The following morning, Judy and Roger have a disagreement over how long Christopher can stay with them. To their surprise, Christopher states that he doesn’t plan to stay long since he wants to return to Swindon to take his A-level maths exams. Later, Judy tells Christopher that she has arranged for him to take the A-level next year instead, which upsets him greatly, causing him to scream in anguish. He is so distressed that he refuses to eat anything.
In drunken anger, full of resentment about Christopher taking Judy’s attention, Roger attacks him verbally and then physically. As a result, Judy leaves Roger, and takes Christopher back to Swindon, living in the family home while Ed moves out to stay with a friend. When Judy takes Christopher to school, they encounter Mrs Shears, who yells insults at Judy. Despite the turmoil, it is arranged for Christopher to take his A-level maths exam that year as initially desired.
At school, Christopher struggles with the first section of the A-level paper due to extreme fatigue, which causes him to panic when he finds himself unable to answer a question. With Siobhan’s assistance, he manages to calm down and successfully finish the paper.
Ed comes to visit to ask about Christopher’s performance in the exam. Although cautious, Christopher shares that his exhaustion might have affected his performance. Ed is pleased to have had some response from him, and leaves.
Through conversations with Siobhan, we find out that Christopher is living with his mother in a small flat. However, he spends time at Ed’s place while waiting for her to come back from work, a situation he finds uncomfortable due to lingering fear of his father. During an arranged meeting, Ed asks Christopher to trust him again, and gives him a present: a dog, which will stay at Ed’s house.
The relationship between Christopher and his father improves, and Christopher shares with Siobhan that they have started cultivating a vegetable garden together. Also, he plans to pursue an advanced maths course next year and aspires to attend university and become a scientist in the future. His solo trip to London has equipped him with the confidence to achieve these goals.