Andrew Wyke, a successful writer of crime fiction, who lives in a large country manor house filled with elaborate games and automata, invites his wife’s lover Milo Tindle, a hairdresser of Italian heritage, to his home to discuss the situation. Andrew explains that he has tired of his wife and wants Milo to take her off his hands. In order to provide Milo with enough money to take care of her, Andrew suggests that Milo steal some jewellery from the house, with Andrew recouping his losses through an insurance claim. Milo agrees and allows Andrew to lead him through an elaborate charade to fake the robbery. At the conclusion, Andrew pulls a pistol and reveals that the entire plot was meant to frame Milo as a robber, giving Andrew an excuse for shooting him. Andrew then appears to execute Milo by shooting him in the head.
A few days later, a policeman, Inspector Doppler, arrives at the manor house to investigate Milo’s disappearance. Andrew at first purports to know nothing, but his guilt becomes evident as the inspector collates clues. Frightened, Andrew breaks down and explains the burglary ruse, but insists that he only pretended to shoot Milo using a blank cartridge and that his rival left the house humiliated but alive. Andrew insists that he has no knowledge of what happened to Milo after he left the house. After finding more seemingly unmistakable evidence that a murder has taken place recently in the house, Doppler arrests Andrew for murder. As Andrew is about to be taken to the station, Doppler reveals himself to actually be Milo, in disguise, having engaged in the deception to get revenge on Andrew.
Just as the score seems to be even between the two, Milo explains that they will now play another game, this time involving a real murder. Milo describes how he visited Andrew’s mistress, Téa, that afternoon, and strangled her. The police will soon be arriving and he has planted evidence throughout the house that could well incriminate Andrew in Téa’s murder. Andrew dismisses this, but phones Téa anyway. Téa’s flatmate, Joyce, tells him tearfully that Téa has been strangled and her body found. Andrew now hunts through the house in an increasing fervour, searching for each piece of evidence on cryptic clues from Milo, who is revelling in Andrew’s predicament. Andrew finds the last item just as Milo sees the police arriving outside the house. Milo answers the door to the police while a dishevelled Andrew straightens himself up. In the background we hear Milo talking to the police officers in an attempt to stall their entry into the house, which Andrew pleaded with him to do. Milo then invites the officers in. However, there are no policemen. Milo reveals that he faked Téa’s death with Joyce and Téa’s willing help, thus fooling Andrew a second time.
Milo gets ready to leave, but he continues to taunt Andrew with humiliating information he obtained from both Andrew’s wife and his mistress. Andrew trains a gun on Milo and threatens to kill him. However, Milo warns him that he has anticipated this reaction and really has called the police, who are due any time. If Andrew kills him then he will be caught red-handed. Andrew, pushed too far when Milo ridicules his detective hero, refuses to believe any of it and shoots Milo, mortally wounding him. The police arrive and approach the house and a distraught and defeated Andrew locks himself in the house. As Milo dies, he tells Andrew to be sure to tell the police that it was “all just a bloody game”; his dying fingers press the automata control box leaving Andrew surrounded by his functioning toys as the police try to enter the house.