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Charles Salt and Christopher Pepper, owners of a Soho gambling club, find themselves in trouble when a man and a Chinese woman are murdered on their premises. Implicated in the killings by an anonymous tipster, Salt and Pepper are questioned by Scotland Yard Inspector Crabbe, who has often harassed them for minor violations at their club. Finally released, they are kidnaped by Colonel Balsom of the British Secret Service. After informing them that the murdered woman was a British agent, the colonel also questions the two men, but to no avail. When Salt and Pepper find the woman's diary which contains the names of four men marked for murder, they decide to solve the case themselves. Despite their efforts, three of the men are murdered, and an attempt is made on their own lives by Colonel Woodstock, a madman who is masterminding a plot to overthrow the British government by threatening nuclear warfare. Salt and Pepper discover this plot when they are abducted and taken aboard a landlocked submarine. They escape and go to Colonel Balsom; but the three men return only to find the submarine gone, and Balsom begins to doubt their sanity. Later, at their club, Marianne, who is a party to the conspiracy, is about to kill Pepper when Salt bursts in and shoots her. Uncovering the conspirators' secret headquarters at the War Museum, Salt and Pepper commandeer an ancient army tank and manage to dispose of the enemy. The two heroes are then brought to Buckingham Palace where they are knighted by the queen.