See How They Run

Dennis, the theater usher, is revealed as the killer of both Kopernick and Mervyn. He commits the murders because he believed the play "The Mousetrap" was based on his brother's tragic death and felt that his family's pain was being exploited for profit.

Earlier scenes show Dennis working in the theater with a haunted expression, hinting at his deeper connection to the production.

Dennis's brother died in a real tragedy that inspired "The Mousetrap." Over the years, he watched the story transform into a successful play and film, feeling that his grief was being trivialized and profited from. When director Kopernick added violent elements to the climax, Dennis saw it as the final insult.

Dennis killed Mervyn because he was involved in developing the film alongside Kopernick and shared ideas that Dennis felt further distorted his brother's story. Mervyn wasn't the primary target but became a victim due to his involvement in the project.

Agatha Christie attempts to intervene by offering Dennis poisoned tea to calm him down and prevent further violence. When that fails, she becomes hostage during the shootout until Stoppard shoots Dennis.

Christie's presence throughout the investigation builds toward her direct involvement in the climax.

The detectives uncover Dennis's motive by analyzing clues he inadvertently left behind. They realize his deep personal connection to the original tragedy and understand that he felt betrayed by how his family's pain was being turned into entertainment.

Dennis holds Agatha Christie hostage during a tense confrontation and threatens to kill others and himself. Detective Stoppard ultimately shoots Dennis when he refuses to release Christie, ending the threat.

Stoppard addresses the audience directly, asking them not to reveal who the killer is. This is a playful nod to classic whodunit conventions, emphasizing that the mystery should remain secret just like in "The Mousetrap," which traditionally doesn't reveal its killer.

The ending is ultimately happy for the main characters. Both Stoppard and Stacker achieve professional success and rekindle their bond. However, Dennis's story adds tragic weight, questioning the cost of artistic exploitation.

Christie's dinner at her house symbolizes that beneath the glamour and theatricality of the entertainment world, real pain and unresolved trauma can still lie hidden. It represents the film's theme that artistic expression can sometimes mask deeper human suffering.

The ending questions whether artists have the right to use real human suffering for creative expression and profit. Dennis's tragic story demonstrates how misusing personal trauma can lead to devastating consequences, leaving viewers to ponder the blurred boundaries between fiction and reality.