In contrast to the group's first film, And Now for Something Completely Different, a compilation of sketches from the first two television series, Holy Grail was composed of new material, and is therefore considered the first "proper" film by the group. It generally parodies the legend of King Arthur's quest to find the Holy Grail. The film was a success on its initial release, and Idle used the film as the inspiration for the 2005 Tony Award-winning musical Spamalot.
Monty Python and the Holy Grail loosely follows the legend of King Arthur. Arthur along with his squire, Patsy, recruits his Knights of the Round Table, including Sir Bedevere the Wise, Sir Lancelot the Brave, Sir Robin the Not-Quite-So-Brave-As-Sir-Lancelot, and Sir Galahad the Pure. On the way Arthur battles the Black Knight who, despite having all his limbs chopped off, insists he can still fight. They reach Camelot, but following a song-and-dance cutaway, Arthur decides not to enter, as "it is a silly place". They are instructed by God (represented by an animated photograph of cricket figure W. G. Grace) to seek out the Holy Grail.
Their first stop is a French-controlled castle where they believe the Grail is being held. After being insulted in mangled Franglais, they try to sneak into the castle in a Trojan Rabbit, but this plan goes terribly wrong when they forget to hide inside it first. The error is compounded when the rabbit is subsequently catapulted back at them. Arthur then decides the group should separate to seek the Grail.
Concurrently, in a manner of "breaking the fourth wall", a modern-day historian is describing the Arthurian legend for a television program. He is suddenly killed by a knight on horseback, triggering a police investigation.
Each of the Knights encounters various perils on their quest. Arthur and Bedevere attempt to satisfy the strange requests of the dreaded Knights who say Ni. Sir Robin narrowly avoids a fight with the Three-Headed Giant by running away while the heads are arguing, causing embarrassment as his minstrel sings "Brave Sir Robin ran away". Sir Lancelot accidentally assaults a wedding party at Swamp Castle, mistakenly believing them to be holding a lady against her will, only to discover an effeminate prince. Galahad is led by a Grail-shaped beacon to Castle Anthrax, populated only by women who wish to perform sexual favours for him, but is "rescued", somewhat against his will, by Lancelot.
The Knights regroup and travel to see Tim the Enchanter (Cleese), who points them to caves where the location of the Grail is written on the walls. To enter the caves, the group is forced to defeat the Rabbit of Caerbannog using the Holy Hand Grenade of Antioch. They enter the cave and are attacked by the Legendary Black Beast of Aaaaarrrrrrggghhh, which devours Brother Maynard (Idle). Arthur and his Knights flee and barely escape by virtue of the beast's animator suffering a fatal heart attack.
With their final destination known, the group travels to its last peril, the Bridge of Death, where each Knight is forced to answer three questions by the bridge-keeper (Gilliam) before they can cross the Gorge of Eternal Peril. Lancelot easily answers the questions and crosses the bridge, but then Sirs Robin and Galahad fail and are thrown into a chasm below. Arthur, at his turn, accidentally tricks the bridge-keeper, who is thrown into the chasm himself.
Lancelot becomes separated from Arthur and Bedevere, and is later shown being arrested by police for the murder of the historian. Arthur and Bedevere travel to the Grail castle, which turns out to be occupied by the same French forces who insulted and drove them off earlier. The Knights amass a large army and prepare to storm the castle, but just as they begin to charge, the modern police arrive on the scene. Arthur and Bedevere are arrested, and one of the officers knocks the film out of the camera, putting an abrupt end to the movie. There are no end credits, only several minutes of organ music.
- Graham Chapman -as- King Arthur / Voice of God / Middle Head / Hiccoughing Guard
- John Cleese -as- Second Swallow-Savvy Guard / The Black Knight / Peasant 3 / Sir Lancelot the Brave / Taunting French Guard / Tim the Enchanter
- Eric Idle -as- Dead Collector / Peasant 1 / Sir Robin the Not-Quite-So-Brave-as-Sir Launcelot / First Swamp Castle Guard / Concorde / Roger the Shrubber / Brother Maynard
- Terry Gilliam -as- Patsy / Green Knight / Old Man from Scene 24 (Bridgekeeper) / Sir Bors / Animator / Gorrilla Hand
- Terry Jones -as- Dennis's Mother / Sir Bedevere / Left Head / Prince Herbert / Cartoon Scribe (voice)
- Michael Palin -as- First Swallow-Savvy Guard / Dennis / Peasant 2 / Right Head / Sir Galahad the Pure / Narrator / King of Swamp Castle / Brother Maynard's Brother / Leader of The