In the bustling heart of London’s West End in 1953, the story begins with [Adrien Brody](slug) portraying the ambitious American director Leo Kopernick, who introduces the audience to the scene. The theater is alive with excitement as a play based on Agatha Christie's famous mystery The Mousetrap is nearing a significant milestone — its 100th performance. This production features distinguished actors such as [Harris Dickinson](slug) as Richard Attenborough and [Pearl Chanda](slug) as Sheila Sim, who deliver compelling performances that captivate the audience.
Kopernick’s primary goal is to persuade the production’s producer, [Reece Shearsmith](slug) as John Woolf, to give him the opportunity to direct a film adaptation of the classic play. However, his attempts to ingratiate himself clash with the strong personalities on stage; he quickly finds himself on the outs with the legendary Richard Attenborough and the play’s writer, [David Oyelowo](slug) as Mervyn Cocker-Norris. Their interactions escalate to comedic and tense heights, with Kopernick even engaging in a physical altercation and crashing into a cake during a heated argument.
Later, while washing off the chaos in a theater restroom, Kopernick notices an eerie, shadowy figure at the window, only to realize it is just a mannequin. Moments later, his nightmare takes a deadly turn when an unknown assailant strikes him with a ski and tries to strangle him. Kopernick manages to break free, but his escape is cut short when he opens a door only to be met with a brick wall. The attacker then uses a sewing machine as a brutal weapon, delivering a fatal blow to Kopernick's head, ending his life in a shocking and violent manner.
The scene shifts to the theater where [Saoirse Ronan](slug) stars as Constable Stalker, a determined rookie officer eager to excel, and [Sam Rockwell](slug) as Inspector Stoppard, who arrives to lead the investigation. They gather everyone present in the theater, including the theater owner Petula Spencer and Woolf’s wife Edana Romney. The officers quickly learn that Kopernick’s murder is not an isolated incident—most of the cast and crew had reasons to dislike him, fueling suspicion among the assembled suspects. With tensions rising, Stoppard announces that the theater will remain closed until the killer is caught and warns everyone there could be more violence.
As the investigation unfolds, Stoppard and Stalker delve into Kopernick's recent activities and relationships. They visit the hotel where Kopernick stayed, speaking with a bellhop who overheard an argument between Kopernick and Mervyn Cocker-Norris, with Mervyn even threatening to kill Kopernick. Flashbacks reveal Mervyn’s discontent over the proposed film adaptation — he disagreed with Kopernick’s vision for a more thrilling, action-oriented ending that involved a detective’s partner being killed after taking a bullet for him. Mervyn also mentions seeing a "plain woman" arriving at the hotel with Kopernick's illegitimate son, adding a mystery element.
Meanwhile, at Shepperton Studios, it’s shown that Woolf is involved in an affair with his assistant [Pippa Bennett-Warner](slug) as they share intimate moments in public. The investigation leads the detectives to Woolf himself, who admits Kopernick approached him at a nightclub, hinting at fears Kopernick might threaten to expose Woolf’s affair with Ann Saville. The players' relationships and motives become more tangled as the detectives interview various characters, including Petula Spencer and her mother, who provide insights into the theater’s behind-the-scenes dealings and Woolf's financial stakes in the "Mousetrap" film rights.
Throughout their inquiries, Stoppard and Stalker share personal stories, revealing their wartime scars—Stalker lost her husband, and Stoppard was injured and divorced after infidelity. One evening, their conversations are interrupted by news articles and photos linking Stoppard to his ex-wife, Joyce, whom they initially believe could be connected to the murders, since she matches the description of a woman seen with Kopernick’s illegitimate son.
In a twist of fate, they attend a performance of The Mousetrap where suspicion continues to simmer below the surface. During a scene depicting murder, Stalker sneaks out, sensing something amiss, and begins to follow a suspicious figure. Her pursuit leads her to Mervyn, who is strangled to death — a brutal parallel to the play they are watching. Meanwhile, Stoppard encounters a mysterious limbo where he sees Kopernick behind a bar, asking if anyone mourns him. When he peeks through a door at the venue, he witnesses disturbing sights, including Mervyn’s body being stuffed by a taxidermist.
Things spiral further when police arrest Stoppard in a cell, suspecting him of the murders. They are convinced that his ex-wife Joyce might be involved, especially after she is detained and questioned. However, she denies being the woman in question, and evidence points away from Stoppard, who begins to suspect Gio, Mervyn’s hot-headed nephew, as a potential killer motivated by hatred and unresolved issues.
The climax builds at Agatha Christie's residence, where a tense gathering occurs. Petula, her mother, Woolf, Sheila, and Ann are present when they realize the killer, Dennis Corrigan, who played a tragic role in childhood neglect and the death of his brother, has returned seeking vengeance against the exploitation of his family’s tragedy. Dennis’s motive is rooted in his desire to halt the production of the film, which he believes exploits his brother’s suffering. A confrontation ensues, culminating in a dangerous standoff amidst chaos created by Fellowes, the butler, who attempts to prevent the chaos but ends up drugged.
As tensions reach a breaking point, Dennis aims his rifle at everyone, but [Laura Morgan](slug) as Joyce intervenes. In a heroic act, [Sam Rockwell](slug) as Stoppard rushes in and gets shot in the process, saving others. [Saoirse Ronan](slug) as Stalker steps into the fray and pushes Stoppard out of the line of fire—mirroring one of the film’s key moments. Stoppard disarms Dennis and shoots him twice, while [Sian Clifford](slug) as Edana kills him with a shovel, ending the threat.
The story concludes with the aftermath of the chaos. Kopernick narrates that Stalker successfully passes her sergeant exam, Stoppard receives a medal for bravery, and Scott is knighted for his efforts. The film closes with a scene of the heroes returning to The Mousetrap, where Stoppard, addressing the audience, comments on the unpredictability of life with a wink: > "This was a real Leo Kopernick ending." To finish, Stalker asks him to quiet down, as she has missed the ending of the play once again. The story emphasizes themes of deception, obsession, and heroism, woven through a complex web of personal and professional revelations—making it a gripping and layered drama that keeps viewers guessing until the very end.
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