Jimmy Logan, a detective, tries to retrieve a diamond from the British crown stolen by the traitorous chief of the English Secret Service. After carrying out dangerous investigations at Montecarlo, Beirut and Hamburg, he discovers that the headquarters of the thieves is located in a mental hospital and sets about infiltrating their ranks.

Jimmy Logan, a detective, tries to retrieve a diamond from the British crown stolen by the traitorous chief of the English Secret Service. After carrying out dangerous investigations at Montecarlo, Beirut and Hamburg, he discovers that the headquarters of the thieves is located in a mental hospital and sets about infiltrating their ranks.