King Arthur is recruiting his Knights of the Round Table throughout England. He is frustrated at every turn by such obstacles as anarcho-syndicalist peasants, a Black Knight who refuses to give up despite losing both his arms and legs, and guards who are more concerned with the flight patterns of swallows than their lord and master. Finally he meets up with Sir Bedevere the Wise, Sir Lancelot the Brave, Sir Galahad the Pure, Sir Robin the Not-Quite-So-Brave-As-Sir-Lancelot, "and the aptly-named Sir Not-Appearing-in-this-Film." They declare themselves the Knights of the Round Table. When 'riding' to Camelot, they are given a quest by God to find the Holy Grail.
They encounter a castle with a Frenchman who randomly taunts them with names like 'Daffy English knnnnnniggits' and odd insults such as, "I fart in your general direction!" and "Your mother was a hamster and your father smelt of elderberries!", together with some mangled Franglais, notably "Fetchez la vache!". The Knights then retreat, weathering a barrage of livestock and executing a poorly thought-out plan to sneak into the castle while concealed within a crudely built giant wooden rabbit which the French throw back at the knights. Arthur decides that he and his knights should search for the Grail individually. After they split up, Sir Robin travels through a forest with his favourite minstrels, and encounters a Three-Headed Giant, Galahad follows a Grail-shaped light to the perils of Castle Anthrax, Sir Lancelot massacres a wedding at Swamp Castle, and Arthur and Bedevere encounter the dreaded Knights Who Say Ni, who demand a shrubbery as tribute. They each overcome or avoid their individual perils in a variety of ways, then reunite to face a bleak and terrible winter, the happenings of which are told in the form of a Gilliam animation. Next they venture further to a pyromaniacal enchanter named Tim, who leads them to a cave guarded by a killer rabbit.
After killing the vicious Rabbit of Caerbannog with the Holy Hand Grenade of Antioch, the knights face the Legendary Black Beast of Aaargh in another animated scene, escaping this peril after the animator suffers a fatal heart attack. Their final task is to cross the Bridge of Death, which is guarded by "the old man from scene 24." Only Arthur, Bedevere, and Lancelot survive the confrontation, but Lancelot mysteriously disappears before the others can catch up to him on the other side. Arthur and Bedevere reach the gates of Castle Aaargh, only to find themselves facing the French taunter once more; the whole quest has in fact been a wild goose chase. As Arthur leads a great army in a charge against the castle, a group of modern police (for the 1970s) suddenly arrive on the scene, disrupting the film's climax. They have been investigating the murder of a "famous historian," who was earlier cut down by an unidentified knight while he was presenting a television program on a topic from the film's supposed era. Lancelot has already been taken into custody, and Arthur and Bedevere are promptly arrested as well. The film ends abruptly when one of the policemen covers the camera lens with his hand, knocking the film loose.