See How They Run

The film opens in London's West End in 1953 with director Leo Kopernick introducing himself and expressing interest in adapting 'The Mousetrap' into a film. The play on stage is celebrating its 100th performance, featuring Richard Attenborough and Sheila Sim in major roles. Kopernick attempts to convince producer John Woolf to let him direct the film.

Kopernick clashes with Richard Attenborough and Mervyn Cocker-Norris, leading to a physical altercation and comedic mishaps such as crashing into a cake. He also goes to the bathroom and notices a suspicious shadow but dismisses it as a mannequin before being attacked.

Kopernick is assaulted by an unknown man who hits him with a ski, attempts to strangle him, and finally bludgeons him with a sewing machine, killing him in the theater. The murder occurs as the cast and crew are present, and his body is left on a stage couch.

Constable Stalker, a rookie officer hoping to impress, and Inspector Stoppard arrive to investigate Kopernick's murder. They gather suspects including Woolf, Sheila Sim, Petula Spencer, and Edana Romney, as they examine the crime scene and suspect multiple motives.

The detectives visit Kopernick’s hotel, interviewing staff who reveal Kopernick argued with Mervyn and discussed a woman arriving with his illegitimate son. Flashbacks show tensions with Mervyn, who disagreed with Kopernick’s film ideas, and a mysterious woman connected to Kopernick emerges.

Inspector Stoppard learns about John Woolf’s affair with his assistant Ann Saville, which he barely hides. Woolf is worried about Kopernick who might expose his relationship, adding a layer of tension to the investigation.

The detectives interview theater staff and visit Attenborough and Sheila, uncovering more about Kopernick’s intentions for the film and Woolf’s financial situation. Personal talks reveal Stalker’s war trauma and Stoppard’s troubled past, raising suspicions about their motives.

Stoppard and Stalker watch the play, and during a murder scene, Stalker follows a suspicious figure. She witnesses Mervyn being strangled and tries to catch the culprit but is unsuccessful. Meanwhile, Stoppard is attacked and knocked unconscious.

Stoppard experiences a surreal limbo state, encountering Kopernick behind a bar who questions if anyone was sad about his death. He then sees a guarded door and a disturbing scene of Mervyn being stuffed by a taxidermist, deepening the mystery.

Stoppard wakes in a cell with Stalker and Scott, accused of murder. A woman claiming to be his ex-wife appears, but he denies her identity. Stalker begins suspecting him due to personal connections and investigates his past relationships.

Petula and others visit Agatha Christie’s home for dinner, discovering that Dennis Corrigan, revealed as the killer, targeted the play and people to stop the adaptation, motivated by a childhood tragedy and personal vendettas. Edana reveals her knowledge of her husband’s affair.

Dennis, armed and dangerous, takes hostages during a tense standoff. Stoppard and Stalker arrive; Stalker is shot while pushing Stoppard aside. A struggle ensues, culminating in Agatha killing Dennis with a shovel, preventing further violence.

Kopernick narrates that Stalker achieved her sergeant exam, Stoppard received a medal, and Scott was knighted for their heroism. The story ends with them returning to watch 'The Mousetrap' live, highlighting the true Leo Kopernick twist in reality.