At the onset of World War II, June McCarthy returns to the United States from Europe, hoping to renew her relationship with Capt. Jimmy Madden. Jimmy is insulted when she urges him to give up his "rustbucket" of a ship for a "real" job, but June remains determined to help him. She enlists the help of Carl Cutler, a businessman she met on her voyage home, and he hires Jimmy's scow for his import-export business. Unknown to June, however, Carl is a German saboteur who has ordered his thugs to kill her because she knows too much. When the assassination fails, Carl takes June hostage aboard Jimmy's ship. After a German submarine intercepts the boat, Carl reveals that he is really a German commander intending to use the ship for the German war effort and, after imprisoning Jimmy and his crew, orders the ship be painted to look like a Danish vessel. Carl then uses the ship to wage a mission of destruction across the Carribean. After June conceives a plan to notify the outside world of the raider, Jimmy regains his trust in her, and together they proceed with the scheme. Using flares cut from life preservers, they send S.O.S. messages in various languages until their flares are intercepted by a British ship, which then comes to the aid of the captured freighter. In a final battle of wits, Carl refuses to surrender to the British and, after releasing his prisoners, goes down with his ship. Rescued by the British, Jimmy and June return together to the United States.