Un comandante alemán de submarinos, el capitán Hardt, es enviado a la islas Órcadas con la misión de destruir la flota británica apostada en Scapa Flow. Allí encontrará la colaboración del matrimonio Blacklock.
Offering a ride to a millionaire, Sam Smith (Leon Errol) agrees to trade places with his passenger for financial reasons. Only when the men in the white coats put the collar on him does Sam realize that the "millionaire" was actually an escaped mental patient.
