Lord Studholme's secretary, Guy Kennion, prepares the invitations for the party that is being given in honour of Princess Amelia of Corsova. He is interrupted by Studholme's daughter, Peggy. Both are worried about what her father will say when he discovers that they have secretly married each other. Studholme appears and insists that Peggy's friend Joan be invited and that Howard Vernon must not come to the party.

Joan has breakfast with her father Sir John, who works at Scotland Yard. He assures himself that she is no longer seeing Vernon as his name has come up during an investigation. Studholome goes to see Vernon, who has fallen on hard times. He asks for the return of the love letters that Joan wrote to him. Vernon badly needs the money and sells them to him for £200, believing that Studholme is acting on Joan's behalf.

Peggy phones her Uncle, General Piddinghoe, to invite him to the party, interrupting him while he takes a bath. He grudgingly accepts. The writer Adrian Chadiott dresses for the party while his wife Anne fusses over him. She brings him the review of his latest book. It is completely negative and Chadiott blames Studholme, as it appeared in one of his papers. When Anne points out that Studholme still invited them to the party, he also places the blame for the bad review on Kennion. Princess Amelia is informed that Studholme has been blocking the attempts to secure a loan, as he is interested in securing for himself the oil concession they are seeking to finance. She takes a gun with her. Before the guests begin to arrive, Studholme threatens to fire his butler unless he returns a cigarette case that he stole. Joan is repulsed when Studholme tries to seduce her. When she rejects him, he produces the letters and says that he will give them to Vernon's wife unless she acquiesces to his advances.

Studholme is furious when he learns that Peggy has married Kennion and says he will disinherit her. Chiddiatt overhears Studholme angrily threatening to blacken Kennion's name. During the party Princess Amelia suggests that they play a game of 'Murder', and pulls out the toy gun she has brought with her. Chiddiatt prepares several strips of paper, on one of which he has written 'murderer'. All the guests draw a piece of paper from a vase. He insists that Sir John must investigate when the lights come on after ten minutes. The butler turns the lights off and in the darkness Sir John learns of the letters when Joan mistakes him for Studholme.

Chiddiatt confronts Studholme about the bad reviews his books are getting in his papers and then leaves when Kennion arrives to 'murder' him for the game. When the lights come back on, Sir John goes to investigate. Becoming bored, the other guests arrive in Studholme's study just after Sir John has retrieved the letters. Studholme is dead. Inspector Ramage takes over the investigation when it becomes clear that Studholme was murdered.

At Scotland Yard, Princess Amelia tells Sir John that she is convinced that it must be suicide, but he tells her that it isn't possible, as Studholme was shot twice, once from a distance of six feet. Piddinghoe claims that he shot him, but it is clear that he is just trying to protect Peggy. Chiddiatt informs the police of the threats that Studholme made against Peggy and Kennion when he heard that they were married. Sir John has throughout the interviews been steering Ramage's questions away from implicating his daughter, but he allows Kennion to be arrested when it looks as though the story of the love letters and Studholme's attempt to blackmail Joan into a liaison might be uncovered.

Kennion is put on trial at the Old Bailey. Vernon admits to having sold Joan's letters to Studholme. Sir John goes on the stand to confess to the murder, but is stopped by Chiddiatt, who produces a gun and shoots at the dock. Chiddiatt admits that he killed Studholme and wanted to incriminate Kennion. He turns the gun on himself and fires.