In the waning days of WWI, a U.S. "Mystery Ship," sets sail for the coast of Spain towing a submarine. Their mission is to find and sink a U-boat that has been especially effective in attacking Allied shipping. Posing as a harmless schooner, the mystery ship is in fact fitted with a formidable gun capable of sinking a U-boat. Stopping in the Canary Islands to refuel, the crew interacts with locals involved with Germans, and with Germans themselves, including the sister of the U-Boat commander, who is lurking offshore waiting for the coming battle.
George O'Brien
as Cmdr. Robert 'Bob' Kingsley
Marion Lessing
as Anna Marie Von Steuben
Mona Maris
as Fraulein Lolita
Walter C. Kelly
as Chief Mike 'Guns' Costello
Warren Hymer
as 'Lug' Kaufman
Steve Pendleton
as Ens. Richard 'Dick' Cabot (as Gaylord Pendleton)
Walter McGrail
as Chief Joe Cobb
Larry Kent
as Lt. 'Mac' McGregor
Henry Victor
as Baron Ernst von Steuben (U-boat commander)
John Loder
as Franz Shiller
