On a Spanish galleon in the year 1687, aristocrat El Nebuloso (Tommy Chong) and his second-in-command, El Segundo (Cheech Marin), abscond with a treasure chest intended for their king. A band of English pirates, led by the dreaded blonde-haired Yellowbeard (Graham Chapman), descend on the ship and seize the treasure. Although his boatswain, Moon (Peter Boyle), planned the attack, Yellowbeard refuses to share the treasure, and as he slams the chest shut, Moon's left hand is severed.Yellowbeard continues his reign of terror, killing more than 500 men, until a betrayal by Moon leads to his 20-year prison sentence for tax evasion in England. Despite years of rehabilitation and torture, Yellowbeard refuses to divulge the location of the treasure.In a jump-forward to 1707, during the final week of his incarceration, Yellowbeard is informed by his wife, Betty (Madeline Kahn), that they have a teenage son named Dan (Martin Hewitt). The pirate is horrified to learn that the boy was raised by Lord Percy Lambourn (Peter Cook) to be an intellectual and a gardener. Dan, however, is fascinated by the idea that his father is the most ruthless pirate of all time.Meanwhile, Commander Clement (Eric Idle) of the Royal Secret Service coerces Queen Anne (Peter Bull) to extend Yellowbeard's sentence an additional 140 years, hoping the pirate will escape and lead authorities to his treasure. As predicted, Yellowbeard breaks out of prison and makes his way to Betty's tavern in search of the treasure map. Betty reveals that she tattooed it onto Dan's head before destroying the original, and directs Yellowbeard to Lambourn Hall, where the boy lives with his mentor.On the advice of informant Harvey "Blind" Pew (John Cleese), Commander Clement follows Yellowbeard into the tavern, but the pirate evades him. Moon's henchman, Gilbert (Marty Feldman), questions Blind Pew about the location of the map, but the informant refuses to assist pirates.Yellowbeard confronts his son at Lambourn Hall, declaring that he needs to remove the boy's head. Dan asks to remain intact and join his father on the treasure hunt. They consult Dr. Ebenezer Gilpin (Michael Hordern), who advises Yellowbeard to pose as a scientist on a voyage of discovery. Gilpin, Dan, and Lord Lambourn sign on as traveling companions. Blind Pew listens outside the office and informs Commander Clement of the group's departure. Moon and Gilbert eliminate Blind Pew by locking him inside a shed full of explosives and detonating them.The next morning, while Yellowbeard wreaks havoc in a brothel, Moon and Gilbert infiltrate a group of sailors and take Dan, Lord Lambourn, and Dr. Gilpin aboard the battleship, Lady Edith. As the ship sets sail, Yellowbeard stows away in the hold, while Captain Hughes lectures the men of their duty in England's war against France. Certain that Yellowbeard is aboard, Moon organizes a mutiny and elects Dan the new captain. Dan places the ship on a course to Madagascar, but during the night, Yellowbeard takes the helm and guides it toward the West Indies.Guided by Yellowbeard's wife, Betty, Commander Clement pursues the Lady Edith. He flies the French flag on his ship to avoid suspicion, and inadvertently attracts cannon fire from the battleship, forcing him to retreat. When the crew of the Lady Edith sights an island, Yellowbeard jumps overboard and swims ashore.Meanwhile, in El Nebuloso's hilltop fortress, an underling named Verdugo demonstrates his latest torture device, designed to break every bone in the human body. To further intimidate victims, it is suspended on a walkway over a pool of acid, to which nothing is impervious except gold and jewels.As the crew arrive on shore, Dr. Gilpin consults the map on Dan's scalp and leads a small party to the other side of the island. El Segundo assumes the landing party is searching for Yellowbeard's treasure and rallies his troops against the invaders, taking Dan prisoner. The boy is placed in the torture device, but refuses to talk.El Nebuloso's daughter, Triola (Stacey Nelkin), begs Dan to lead her father to the treasure, "for the grace and glory of God." When Dan suggests that torture is inconsistent with Christian ideals, Triola explains her father's desire to expedite his victims' entry into heaven. After Dan and Triola declare their love for each other, he promises to find the treasure and return for her, although El Nebuloso may be killed in the process. The avaricious Triola voices her approval.Elsewhere in the castle, El Nebuloso instructs El Segundo to feign resistance as the invaders approach the castle, enabling him to trap their leaders in the torture chamber. Yellowbeard joins Dr. Gilpin and Lord Lambourn as they lead their men into the fortress. Although guards pretend to fall dead at their feet, Yellowbeard proceeds to dispatch all in his path, including El Segundo, who dies a highly dramatic death, and receives a round of applause from his men.Upon entering the torture chamber, Dan's comrades find El Nebuloso threatening the boy with a sword. Yellowbeard swings from a rope and pushes the despot into the acid. Triola enters as Dan is set free, and she is happy to learn that her father is in heaven with all of the people he sent there.Moon leads his band of mutineers into the chamber and a sword fight ensues where Gilbert also falls into the pit of acid. Yellowbeard offers Dan the opportunity to fight Moon, but Triola intervenes and pushes the boatswain into the acid.Returning to the beach, Yellowbeard uses his sword to shave the top of Dan's head, revealing the map. When they unearth the treasure, Dan embraces his father, accidentally stabbing him with a dagger. Yellowbeard is proud of his son for adhering to the family tradition of patricide. Clement confiscates the treasure in the name of Queen Anne, prompting Triola to reject Dan and declare her love for the commander.Later, while Commander Clement considers keeping the treasure for himself, Dan and Yellowbeard burst into his cabin and take control of the ship. Triola has renewed affection for Dan, and Yellowbeard declares, "There's pirating to be done."