Robert Maynard (Keith Andes) is out to prove that privateer Sir Henry Morgan (Torin Thatcher) is a pirate, and goes undercover on another pirate ship in hopes of getting details of Morgan's crimes. But the ship he chooses is captained by the ruthless Blackbeard (Robert Newton), who killed the previous captain and took his bride-to-be, Edwina (Linda Darnell), hostage. Now Maynard must fight to free himself and Edwina from both Blackbeard and Morgan, who also wants Edwina for his own.