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.