Christopher Boone finds Wellington—his neighbor, Mrs. Shears’, dog—dead. She calls the police naming Christopher in the crime. When confronted by a policeman, Christopher strikes the officer and is taken into custody. He is issued a citation for the confrontation.
Christopher’s mother, Judy, died several years ago, so he is being raised by his father, Ed. Ed cares for his son and is invested in his education— he wants Christopher to take his A Level Maths test three years early. Ed, however, does not want Christopher to look into Wellington’s death. Christopher goes behind his father’s back and investigates anyway. At the urging of his teacher, Siobhan, he writes a book about his detecting experience (her reading the book aloud becomes narration for the audience).
A nosy neighbor, Mrs. Alexander, tells him that his mother was having an affair with Mr. Shears before her death. He includes this in his book. When his father finds the book he is enraged. He hits Christopher and takes it away. While searching for his book Christopher discovers letters to him from his deceased mother. He realizes his father had lied to him and his mother, Judy, is alive and living with Mr. Shears in London.
Ed confesses to hiding Judy from Christopher and also to killing Wellington in anger. Fearful for his safety, Christopher decides to travel alone to London to find his mother.
Siobhan asks if the school can make a play out of Christopher’s book (the play the audience is watching).
Christopher takes his pet rat, Toby, along on his journey. He steals Ed’s cashpoint (ATM) card and makes his way to the train station where he is overwhelmed and overstimulated by the environment. With help from a policeman Christopher starts his journey— but not without obstacles and melt-downs along the way. On the last leg of the journey, Toby escapes on the tracks of the “Tube” (the London Underground train system). He attempts to rescue him but ends up being rescued himself.
Finally reunited with his mother (and Mr. Shears with whom she is living) Christopher tells her that he has not replied to her letters because he thought she was dead. Furious at this information Judy assures Christopher he can stay with her. Desperate to find Christopher, Ed finds his way to Judy’s apartment where he and Judy fight over Christopher.
Who will Christopher live with? Will he be able to reconcile his father’s actions and his mother’s absence? Will he be able to take his A Level Maths test?