In the eighteenth century Caribbean, Capt. Vallo, known as the "Crimson Pirate," captures a Spanish galleon bearing the king's emissary, Baron Gruda, along with guns and ammunition intended to suppress a people's rebellion on the islands. Vallo's men are disappointed to find there is no gold in the booty, but Vallo schemes to sell the guns to the rebel leader, El Libre, and instead of killing Gruda, turn El Libre in to him for an additional ransom. The first mate, Mr. Humble Bellows, balks, as these dealings seem too legitimate and therefore undignified for a pirate. Nevertheless, Vallo's crew sails for the Isle of Cobra to find El Libre. Vallo and his mute sidekick Ojo go ashore and find Pablo Murphy, who they suspect is part of the rebellion, and when he is unwilling to trust them, they prove their loyalty by brawling with the king's soldiers. After Pablo helps them escape, they introduce themselves and try to strike a bargain, but the rebels take them prisoner intending to hold them hostage for the guns. Vallo and Ojo are freed with the help of El Libre's daughter Consuelo, but she requires in return that they help her free El Libre, who is imprisoned on the island of San Perro. With Vallo posing as Gruda, they sail to San Perro on the stolen Spanish galleon and request, in the name of the king, that El Libre be transferred to them. The governor, eager to socialize with a new face, invites Vallo to dinner, so Vallo, Consuelo and Ojo go ashore, but after dinner, they are recognized by a dancing girl, whom the pirates had previously caught. Vallo and his cohorts make a hasty exit and escape with El Libre and Elihu Prudence, a scientist imprisoned as a traitor. Vallo finds himself attracted to Consuelo, who is grateful to him, until Bellows discloses Vallo's plan to turn them over to Gruda. Vallo tries to sneak Consuelo, El Libre and Prudence off the ship, but Bellows, who considers such gallantry beneath the dignity of a pirate, stirs up a mutiny. Vallo, Ojo and Prudence are chained in a small boat to die on the sea, but with the help of Prudence's scientific knowledge, manage to get to shore. Meanwhile, Gruda, who has been double-crossing Vallo with Bellows, captures El Libre and Consuelo, and then double-crosses Bellows by ordering the soldiers to overtake the ships and imprison the pirates in a net. On shore, Gruda announces that Vallo is dead and forces Consuelo to marry the governor or see her father killed. For a wedding present, the peasants are ordered to fill a barn with grain. However, Vallo rallies the people for an attack, while Prudence sets to work making nitroglycerin and other inventions that he has read about in theoretical treatises. On the day of the wedding, the governor is told that it is the custom of the people to hold a folk ceremony. Vallo, Ojo and Prudence, dressed as peasant girls, try to walk up to the podium and free Consuelo, but El Libre unexpectedly arrives armed and accidentally blows their cover. Vallo, Ojo and Prudence escape to a barn, where a hot air balloon, designed by Prudence, is hidden and follow Gruda as he escapes to the king's ship with Consuelo. Vallo and Ojo then jump onto the pirate ship and free the pirates. Knowing that the king's men will soon shoot them broadside, the pirates sneak off their own ship and swim toward the king's ship, excepting Bellows, who elects to die like a pirate. After Gruda destroys the pirate ship and his men relax in celebration, the pirates jump aboard and overtake them. Vallo rescues Consuelo from Gruda's clutches, and soon after, Prudence shows up in a submarine that he designed, but it is not needed, as Vallo and his men have already won the battle.