NOTE: This spoiler was submitted by Jeremy

London West End, 1953 – Hotshot American director Leo Kopernick (Adrien Brody) narrates the introduction. A play based on Agatha Christie’s “The Mousetrap” is continuing a historic run, celebrating 100 performances and featuring renowned actor Richard Attenborough (Harris Dickinson) and his wife Sheila Sim (Pearl Chanda) in major roles. Kopernick has shown up in the hopes of convincing the show’s producer John Woolf (Reece Shearsmith), to allow him to direct the film adaptation. Kopernick butts heads with Attenborough and the play’s writer, Mervyn Cocker-Norris (David Oyelowo), going as far as getting into a fight with Attenborough and crashing into a cake.

Kopernick goes to the bathroom to wash himself off. He sees a suspicious shadowy figure by the window, but it is just a mannequin. Then an actual man whacks Kopernick across the face with a ski. The assailant attempts to strangle Kopernick, but he breaks himself free and tries to run, only to open a door and find a brick wall behind it. The assailant then grabs a sewing machine and bludgeons Kopernick in the head, killing him.

Constable Stalker (Saoirse Ronan) awaits the arrival of Inspector Stoppard (Sam Rockwell) as they gather everyone in the theater as suspects, including the theater owner Petula Spencer (Ruth Wilson) and Woolf’s wife Edana Romney (Sian Clifford). Stalker is a rookie with hopes of acing her sergeant exam and tries to make a good impression on Stoppard. They find Kopernick’s body left on the couch onstage. The two inspect the crime scene and try to find motives and suspects. Most of the people in the room took issue with Kopernick, and Stoppard informs them all that there may be another killing among them, so he opts for the theater to be closed until the murder is solved.

Commissioner Harrold Scott (Tim Key) berates Stoppard for drawing negative attention to the murder and the department after closing the theater. He orders Stoppard to take Stalker on the case to “show her the ropes” of the job.

Stoppard and Stalker go to the hotel where Kopernick was staying and speak to a bellhop who overheard him arguing with Mervyn, with the latter outright saying he will kill Kopernick. The two visit Mervyn and try to understand his background with Kopernick. Flashbacks show that he disagreed with Mervyn’s vision for the play and pitched his own action-packed ending for “The Mousetrap” film adaptation, involving the lead detective’s partner being killed by the killer after taking a bullet for him. Mervyn mentions seeing a “plain woman” arriving at the hotel with Kopernick’s illegitimate son. Mervyn is also shown living with his nephew Gio (Jacob Fortune-Lloyd), who is described as hotheaded.

At Shepperton Studios, it is shown that Woolf is engaged in an affair with his assistant, Ann Saville (Pippa Bennett-Warner), which he barely attempts to hide as he tries to kiss her out in public.

The duo stop in an alley as Stoppard claims he is going for a dentist appointment. Stalker spots him walking into a pub and appears disappointed with him. They then visit Woolf, who mentions how Kopernick approached him in a nightclub and became aware of his affair with Ann, and Woolf appeared worried that Kopernick might expose him. After they leave, Stoppard mentions walking into the pub, but Stalker simply tells him he can trust her with anything he needs to tell her. The two go home, where Stalker lives with her two children, and Stoppard lives with his alcohol.

The following day, Scott encounters the duo and says he is desperate for good news. They say they may have a reliable lead. The pair interview a theater usher named Dennis (Charlie Cooper), who claims to have seen a figure acting suspiciously, but it doesn’t provide them with much of a lead. They then go to visit Attenborough and Sheila. She tells the duo that Kopernick offered her the lead role in the “Mousetrap” film. Stoppard and Stalker then go to visit Petula, who is watching her mother (Ania Marson) perform onstage by herself. Petula tells the two that she signed over the film rights of “Mousetrap” to Woolf, who was nearly caught in his affair by Edana. She adds that Woolf would be unable to produce the film adaptation until the show completes its production, meaning he’s stuck there.

Stoppard and Stalker go to a pub, where they talk about their personal lives. Stalker mentions her kids and how she lost her husband in the war, and Stoppard mentions how he was injured in the war and divorced his wife after she was unfaithful to him and had a child with another man. Stalker drives a drunken Stoppard home and comes across some news articles and a picture of Stoppard with his ex-wife, who fits Mervyn’s description of a “homely woman with glasses”. Stalker starts to become suspicious and begins to ask around as to the name of Stoppard’s ex-wife in connection with a book of names they found in Kopernick’s hotel room.

The duo go to a performance of “The Mousetrap” since Attenborough gave them tickets. During the scene where one character is being murdered, Stalker steps out and follows who she thinks is Stoppard. A figure comes across Mervyn and strangles him to death. Stalker chases after the figure but fails to catch him. Meanwhile, the figure whacks Stoppard over the head and knocks him out. He appears in a sort of limbo state where he finds Kopernick behind a bar, asking if anybody was sad about his demise. Stoppard doesn’t respond, which doesn’t surprise Kopernick. Stoppard approaches a door guarded by Dennis, who won’t let him through without a ticket. He peeks through the door and sees what appears to be Mervyn being stuffed by a taxidermist.

Stoppard awakens in a prison cell with Stalker and Scott. They accuse him of murder and say they have found his ex-wife Joyce, supposedly the same woman that Mervyn had seen. Stalker believes Stoppard killed Kopernick because his wife had cheated on him with Kopernick. A woman named Joyce (Laura Morgan) shows up, but Stoppard says that is not HIS Joyce, so he is let go. Stalker escorts Joyce home and asks her questions about when she went to see Kopernick with their son. Joyce mentions hearing a voice with an accent like a village idiot. Meanwhile, Stoppard suspects Gio, since Mervyn had mentioned him to be a taxidermist, and he believes he had a motive to kill both Mervyn and Kopernick. Stoppard ends up finding news articles on two young brothers, one of whom died. Stoppard and Stalker individually head out to find the killer.

Petula arrives at the home of Agatha Christie (Shirley Henderson) along with her mother, Attenborough, Sheila, Woolf, and Ann. The stuffy butler Fellowes (Paul Chahidi) attempts to deny them entry despite an invitation, but another guest, Max Mallowan (Lucian Msamati), allows them in. They were supposedly invited for a dinner, but they find that Agatha is supposedly tied up by the killer….Dennis. Petula’s mother recognizes him as Dennis Corrigan, whose younger brother died when they were children due to neglect. He wanted to stop the play from continuing because he thought it was an exploitation of his tragedy, so he killed Kopernick to stop the film from being made and Mervyn for writing it. Dennis kicks the rug where “Agatha” is tied up, but it is actually Edana, who reveals she already knew about her husband’s affair.

Just as Stoppard and Stalker get closer to the house, Agatha emerges with cups of tea, one of which is meant to drug Dennis while Fellowes attempts to grab his rifle. It backfires, Fellowes faints from the drugged tea, and Dennis aims the gun at everyone. Stoppard shows up and has his tires shot out. He runs into the house and faces Dennis, but before Dennis fires, Stalker runs in and pushes Stoppard out of the way (like in Kopernick’s ending). She gets a nick, but Stoppard is shot. He shoots Dennis twice before Agatha kills him with a shovel to the head. Stoppard is taken to the hospital.

In the epilogue, Kopernick narrates that Stalker aced her sergeant exam, Stoppard was awarded a medal for his heroics, and Scott was given a knighthood. Stoppard and Stalker return to watch “The Mousetrap”. As Stoppard addresses the audience, Stalker asks him to be quiet because she missed the ending last time. Kopernick says that this was a real Leo Kopernick ending.

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In 1953 London, a performance of "The Mousetrap," based off of Agatha Christie's story, is marred by the murder of American filmmaker Leo Kopernick. Inspector Stoppard and Constable Stalker get on the case as they interview those who knew (and did not like) Kopernick, including playwright Mervyn Cocker-Norris and acclaimed actor Richard Attenborough. Stalker begins to suspect Stoppard when she learns that his ex-wife had a child with another man, and Kopernick had an illegitimate child with a woman matching a description of Stoppard's ex-wife and a woman seen at Kopernick's hotel.

Mervyn is later killed during a performance of the show, and Stoppard is arrested after he is knocked out by the real killer. After he is proven innocent, he and Stalker find new clues that lead them to find out that the real killer is Dennis Corrigan, an usher at the theater showing "The Mousetrap." The play is based on a tragedy involving his younger brother, who died as a child, and he was angry at the play for exploiting it, so he killed Kopernick to stop him from making a film adaptation. He tricks several people involved in the show into coming to the home of Agatha Christie in an attempt to kill them all, but Stoppard and Stalker arrive in time to stop Dennis, who is shot by Stoppard and bludgeoned with a shovel by Agatha, killing him.

In the end, Stalker aces her sergeant exam, and Stoppard is given a medal for his heroics. The two also become closer as partners and friends.