Late in the 18th century, Caribbean pirate Captain Vallo (Burt Lancaster) and his crew capture a frigate of the King's navy. The ship is carrying Baron Gruda (Leslie Bradley), a special envoy of the King on his way to the island of Cobra to crush a rebellion. Vallo proposes selling the frigate's weapons cache to "El Libre", the leader of Cobra's rebels. Baron Gruda counters by proposing that Vallo capture "El Libre" and bring him to the Baron for a sizable reward. Vallo accepts, and Baron Gruda and his crew are released, while Vallo keeps the frigate. Some of his crew complain that this is not pirate business, but they come around when they find out the large amount of profit to be made.