In 1953 London, Agatha Christie's play The Mousetrap celebrates its 100th performance, and sleazy American director Leo Köpernick has been hired by producer John Woolf to direct a film adaptation. After his drunken behaviour towards the female lead Sheila Sim leads to a fistfight with her husband and co-star Richard Attenborough, Köpernick is killed backstage by an unseen assailant. Inspector George Stoppard investigates the murder, shadowed by the inexperienced Constable Stalker. Everyone at the theatre is declared a suspect and a potential victim, including the play's producer Petula Spencer and Woolf's wife Edana Romney. Stoppard attempts to close the theatre until the murder is solved, but the Metropolitan Police Commissioner, Harold Scott, intervenes, assigning Stalker to assist in the investigation. Stoppard and Stalker search Köpernick's room at the Savoy Hotel, finding his address book of women's details. The hotel manager recalls Köpernick's violent argument with screenwriter Mervyn Cocker-Norris, who admits to arguing with Köpernick over the director's vision for an action-packed ending for The Mousetrap film. Mervyn, who lives with his hot-headed "nephew" Gio, remembers a "plain woman" arriving at the hotel with Köpernick's son. Stoppard and Stalker question Woolf who was blackmailed by Köpernick after he discovered Woolf's affair with his assistant, Ann. Dennis, an usher at the theatre, offers an unhelpful description of a suspicious figure; Attenborough and Sim are questioned as well. Spencer explains that she optioned The Mousetrap's film rights to Woolf, but he would be contractually unable to produce the adaptation until the show had closed. At a pub, Stoppard bonds with Stalker and reveals that his unfaithful wife left him after having a child with another man. Driving a drunken Stoppard home, Stalker finds news articles and a picture of his ex-wife, fitting Mervyn's description of a "homely woman with glasses". Combined with Dennis's description, this leads Stalker to suspect Stoppard may be the killer, and she searches for his ex-wife in Köpernick's address book.