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, recounts receiving Excalibur from the Lady of the Lake, defeats the Black Knight and observes an impromptu witch trial. He recruits Sir Bedevere the Wise, Sir Lancelot the Brave, Sir Galahad the Pure and Sir Robin the Not-Quite-So-Brave-as-Sir-Lancelot, 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 Englishmen, driving them back with a barrage of barnyard animals. Bedevere concocts a plan to sneak in using a Trojan Rabbit, but no one hides inside it, and the Englishmen are forced to flee when the rabbit 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 a knight on horseback, triggering a police investigation.
On the knights' travels, 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. Sir Galahad is led by a grail-shaped beacon to Castle Anthrax, which is full of young women, but is unwillingly "rescued" by Lancelot. Lancelot receives an arrow-shot note from Swamp Castle. Believing the note is from a lady forced to marry against her will, he storms the castle and slaughters several members of the wedding party, only to discover the note was from an effeminate prince.
Arthur and his knights regroup and are joined by three new knights, as well as Brother Maynard and his monk brethren. 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 its power, the knights attack the Rabbit, which easily kills Sirs 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.
An animated cave monster devours Brother Maynard, but Arthur and the knights escape after the animator dies of a heart attack. The knights approach the Bridge of Death, where the bridge-keeper challenges them to answer three questions to pass, or else be cast into the Gorge of Eternal Peril. Lancelot easily answers the questions and crosses. Robin is defeated by an unexpectedly difficult question, and Galahad fails an easy one; both are flung into the gorge. When the bridge-keeper attempts a difficult question about swallows on Arthur, he turns the question on the bridge-keeper, who cannot answer and is thrown into the gorge.
Arthur and Bedevere cannot find Lancelot, unaware that he has been arrested by police investigating the historian's death. Arthur and Bedevere reach Castle Aarrgh, but find it occupied by the French soldiers. 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, and break the camera, ending the film.
Graham Chapman as Arthur, King of the Britons
Chapman also provides the voice of God, among other roles
John Cleese as Sir Lancelot the Brave
Cleese also plays the Black Knight, French Taunter, and Tim the Enchanter, among other roles
Terry Gilliam as Patsy, Arthur's Servant
Gilliam also plays the Soothsaying Bridgekeeper, among other roles, and appears as himself as the Weak-Hearted Animator
Eric Idle as Sir Robin the Not-Quite-So-Brave-as-Sir-Lancelot
Idle also plays Concorde, Lancelot's squire, Roger the Shrubber, and Brother Maynard, among other roles
Terry Jones as Sir Bedevere the Wise
Jones also plays Prince Herbert, among other roles
Michael Palin as Sir Galahad the Pure
Palin also plays the Leader of the Knights Who Say Ni, Lord of Swamp Castle among other roles, and provides the voice of the film's narrator
Connie Booth as Miss Islington, the Witch
Carol Cleveland as Zoot and Dingo, identical twin sisters
Neil Innes as the Leader of Robin's Minstrels, among other roles
Bee Duffell as the Old Crone
John Young as Frank the Historian
Rita Davies as Frank's Wife
Avril Stewart as Dr. Piglet
Sally Kinghorn as Dr. Winston
Mark Zycon as Sir Robin (stand-in)
Sandy Johnson as a Knight Who Says Ni
Julian Doyle as Police Sergeant
Richard Burton as One-Legged Black Knight (stand-in)
Roy Forge Smith as Inspector End Of Film
Maggie Weston as Page Turner
Charles Knode as Camp Guard