On a ship docking in Port Said is Madame Delacour (Margaret Irving), wife of the French naval admiral.
Delacour and her daughter Marie (Joan Carroll) are befriended by the charming Eric Norvel (George Sanders), the goofy Rollo Venables (Robert Coote), and someone posing as Mr. Moto (Teru Shimada).
Norvel takes Delacour and Venables to a variety show featuring Fabian the Great (Ricardo Cortez), a ventriloquist.
Fabian enlists his girlfriend Connie (Virginia Field), who is unaware he is an agent, to follow Mr. Moto the next day.
She sees Moto visit the Port Commandant's office where he learns of the salvage ship "The Vulcan" captained by Hawkins.
Fabian lures Burke to "The Vulcan" and reveals his plan to blow up the French fleet and blame the British.
Norvel dives down to await a signal from Fabian but Moto overpowers him and prematurely detonates the explosives meant to destroy the French fleet.
Moto discovers the saboteur's plans in Fabian's dummy but never reveals to the audience which country tried to engineer a war between France and England.
The New York Times thought the "method" used by Moto was "a little tough on the audience" but praised the "rousing old fashioned climax".