Pirate Captain Vallo captures the King's ship carrying His Majesty's envoy, Baron Gruda, on his way to crush the rebellion and destroy El Libre.

Vallo proposes that they join forces to earn a fortune for the Captain and his buccaneer crew by infiltrating the rebels and betraying them for the reward.

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, a special envoy of the King on his way to the island of Cobra to crush a rebellion led by a man known as El Libre.

Vallo and his crew sail to Cobra, where the captain and his lieutenant, Ojo, go ashore and meet with the island's rebels, led by Pablo Murphy and El Libre's daughter Consuelo.

In order to defeat the well trained and well armed troops on Cobra, the professor has the rebels build a variety of futuristic weapons, such as tanks, Gatling guns, flamethrowers, a hot air balloon, and a submarine.

H. Weiler of The New York Times described the film as "a slam-bang, action-filled Technicolored lampoon ... Any viewer with a drop of red blood in his veins and with fond memories of the Douglas Fairbanks Sr. school of derring-do should be happy to go on this last cruise of the crimson pirate.

Its tongue-in-cheek treatment pokes fun at pictures of this type, and for that reason it should be enjoyed, not only by the action fans, but also by others who are willing to accept it for the good-natured spoof that it is.

"[9] The Monthly Film Bulletin wrote, "The jokes are as unsophisticated as the adventure, and the combination of violence and slapstick makes for quite good fun.