Captain Hugh Diamond, a Special Branch officer, tracks an agent of a South American monarchist party on a ship from Tangiers bound for London. The agent, Felipe Nagy, and his female accomplice plan to steal the Crown of Destiny from the Trafalgar museum, where it is on show. They plan then to return it to a South American republic and exploit it to destabilise the country. When Diamond is knocked unconscious by Nagy, the latter succeeds in stealing the crown. His plot to smuggle the crown out of the country by helicopter is foiled by Diamond with the help of Ronnie, a photographer.
—GusF