In AD 932, King Arthur and his squire, Patsy, travel Britain searching for men to join the Knights of the Round Table. Along the way, Arthur debates whether swallows could carry coconuts, passes through a town infected with the plague, recounts receiving Excalibur from the Lady of the Lake to two anarcho-syndicalist peasants, defeats the Black Knight, and observes an impromptu witch trial. He recruits 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, along with their squires and Robin's minstrels. Arthur leads the knights to Camelot, but, after a musical number, changes his mind, deeming it "a silly place". As they turn away, God appears and orders Arthur to find the Holy Grail.




Arthur and his knights arrive at a castle occupied by French soldiers, who claim to have the Grail and taunt the Britons, driving them back with a barrage of barnyard animals. Bedevere concocts a plan to sneak in using a Trojan Rabbit, but forgets to tell the others to hide inside it; the Knights are forced to flee when it is flung back at them. Arthur decides the knights should go their separate ways to search for the Grail.




A modern-day historian filming a documentary on the Arthurian legends is killed by an unknown knight on horseback, triggering a police investigation.




Arthur and Bedevere are given directions by an old man and attempt to satisfy the strange requests of the dreaded Knights Who Say "Ni!". Sir Robin avoids a fight with a Three-Headed Knight by running away while the heads are arguing among themselves. Sir Galahad is led by a grail-shaped beacon to Castle Anthrax, which is occupied exclusively by young women, who wish to be punished for misleading him, but he is unwillingly "rescued" by Lancelot. Lancelot receives an arrow-shot note from Swamp Castle. Believing the note is from a lady being forced to marry against her will, he storms the castle and slaughters several members of the wedding party, only to discover the note is from an effeminate prince.




Arthur and his knights regroup and are joined by Brother Maynard, his monk brethren, and three new knights: Bors, Gawain and Ector. They meet Tim the Enchanter, who directs them to a cave where the location of the Grail is said to be written. The entrance to the cave is guarded by the Rabbit of Caerbannog. Underestimating it, the knights attack, but the Rabbit easily kills Bors, Gawain and Ector. Arthur uses the "Holy Hand Grenade of Antioch", provided by Brother Maynard, to destroy the creature. Inside the cave, they find an inscription from Joseph of Arimathea, directing them to Castle Aarrgh.




A cave monster devours Brother Maynard, but Arthur and the knights escape after the animator unexpectedly suffers a fatal heart attack. The knights approach the Bridge of Death, where the bridge-keeper demands they answer three questions in order to pass or else be cast into the Gorge of Eternal Peril. Lancelot easily answers simple questions and crosses. An overly cocky Robin is defeated by an unexpectedly difficult question, and an indecisive Galahad fails an easy one; both are magically flung into the gorge. When Arthur asks for clarification on a question regarding the airspeed of an unladen swallow, the bridge-keeper cannot answer and is himself thrown into the gorge.




Arthur and Bedevere cannot find Lancelot, unaware that he has been arrested by police investigating the historian's death. The pair reach Castle Aarrgh, but find it occupied by the French soldiers from earlier in the film. After being repelled by showers of manure, they summon an army of knights and prepare to assault the castle. As the army charges, the police arrive, arrest Arthur and Bedevere for the murder of the historian and break the camera, abruptly ending the film