Please note, this synopsis contains spoilers.

Christopher Boone is 15 years old and lives in the English town of Swindon, located about 100 miles west of London. He is extremely curious, an excellent mathematics student, and has an extraordinary ability to observe and retain highly detailed information about any situation he finds himself in. Christopher lives with his father, Ed, and shares a close bond with his teacher, Siobhan, who encourages him to keep a written journal of his thoughts and activities, which they read aloud together as the play progresses. As a young person on the autism spectrum, certain social situations can be challenging or confusing for Christopher to navigate, and his work with Siobhan helps him express and process his unique perspective on the world around him.

Our story begins when Christopher discovers his neighbor Mrs. Shears’ dog Wellington has been stabbed to death with a garden fork. Mrs. Shears assumes Christopher is the killer and calls the police. Christopher explains he did not kill Wellington, but the interaction becomes contentious and Christopher ends up arrested for hitting the policeman. Ed comes to the police station to retrieve Christopher. It is sorted out that the teen definitely did not kill the dog, but the incident puts more stress on the already touchy relationship between Ed and Christopher. Christopher resolves to make it his personal mission to solve the case of who killed Wellington. Meanwhile, in sessions with Siobhan, we learn that Christopher’s mother Judy developed heart disease and died in the hospital; Ed relayed the news but never allowed Christopher to visit Judy during her illness.

While Ed works with Christopher’s teachers to get him placed in advanced mathematics classes, Christopher sets out on his investigation. He goes around to the various people of his neighborhood looking for clues. As he gathers information about Wellington’s death, he stumbles into secrets about his own family history. Eventually, Christopher discovers that his mother had an affair with their neighbor, Roger Shears. When Ed learns of his son’s discovery, he unleashes a flood of repressed emotion upon his son, which culminates in a moment of violence. In the aftermath, Christopher discovers that his father has lied to him — Christopher’s mother is alive and well and living in London. She left Christopher and Ed behind, but she has since mailed dozens of letters to Christopher that Ed has kept hidden for years. Ed is forced to tell Christopher the truth about his parents’ relationship, and Ed also confesses to murdering the dog Wellington in a fit of despairing rage. 

Ed’s confession throws Christopher’s world into chaos. Christopher is convinced that since his father killed the dog he might kill him too, so he decides to leave home and find his mother. This journey will be the first Christopher has undertaken alone, and his way to London (via train) leads to a number of exciting but potentially hazardous interactions. Ed ends up catching the determined Christopher at the train station. He tries to stop Christopher, but ultimately watches him set out on his own to find Judy.

When Christopher finally makes it to Judy and Roger’s house unannounced, they are shocked to see him. Judy is happy to reconnect with her child, but Roger is not particularly pleased. Judy discovers that Ed told Christopher she was dead. It’s not easy news to process. Ed soon arrives at the house in pursuit of Christopher, and the boy’s parents argue. Christopher indicates he wants to stay with Judy, not Ed, at least until his advanced maths exams come up the following week. As Judy readjusts to life taking care of Christopher, a life which was always challenging for her, her familiar patterns return and she upsets Christopher by canceling his advanced classes without telling him. Roger does not want Christopher in their lives and becomes violent with him; Judy knows they have to return to Swindon. 

Judy faces the fact that she must place Christopher’s needs in front of her own, and with help from Ed and Siobhan, Christopher’s classes are reinstated and he takes his exams. In the end, the family begins a slow road to reconciliation. Christopher chooses to live with Judy, and his parents will share custody between London and Swindon. Ed gets a puppy, and the dog helps Christopher move toward forgiving his father. In a final moment with Siobhan, Christopher expresses pride at his bravery in solving Wellington’s murder and finding out the truth about his family. He is left wondering about the full extent of his potential and has the sense, perhaps for the first time, that he can accomplish anything.