In 932 AD, King Arthur (Graham Chapman) and his devoted squire, Patsy (Terry Gilliam), embark on a quest across Britain to assemble brave souls to join the Knights of the Round Table. Along this whimsical journey, Arthur engages in a philosophical debate regarding the possibility of swallows carrying coconuts and fondly recalls how the Lady of the Lake bestowed upon him the legendary Excalibur. "People say that executive power cannot come from strange aquatic ceremonies, and only from a democratic system of voting," he muses. The knight encounters various absurdities, including vanquishing the Black Knight who blocks his passage and witnessing a makeshift witch trial, which absurdly decides a woman’s guilt by weighing her against a duck.
To his astonishment, many citizens remain ignorant of their monarch, presuming they exist in a self-governed society reminiscent of modern parliamentary democracy. During his quest, Arthur gathers notable figures, recruiting Sir Bedevere (Terry Jones) the Wise, Sir Lancelot (John Cleese) the Brave, Sir Galahad (Michael Palin) the Pure, and Sir Robin (Eric Idle), who is dubbed the Not-Quite-So-Brave-as-Sir-Lancelot, along with their squires and Robin's minstrels.
As they journey to Camelot, Arthur is momentarily struck by the allure of a musical number but quickly deems Camelot "a silly place" and diverts his attention when a divine command from God entreats him to locate the Holy Grail. Their travels lead them to a fortress manned by French soldiers who claim possession of the Grail, taunting the British knights while unleashing a flurry of farm animals to fend them off. In an attempt to infiltrate the castle, Bedevere devises a ludicrous plan involving a Trojan Rabbit, but without anyone hiding inside it, the Britons find themselves on the receiving end of a comical blunder.
Divided, Arthur encourages the knights to pursue their individual searches for the Grail. A modern historian shooting a documentary on Arthurian legends meets his demise at the hands of an errant knight, triggering a police investigation into this curious affair.
As they wander the land, Arthur and Bedevere receive guidance from an enigmatic old man and face the nonsensical demands of the notorious Knights Who Say "Ni!". Sir Robin cleverly evades a confrontation with a Three-Headed Knight by taking flight while the heads bicker among themselves. Sir Galahad finds himself drawn to Castle Anthrax, guided by a grail-shaped signal, where he encounters a bevy of alluring women, led by the irresistible twin sisters Zoot and Dingo, only to be "rescued" by Lancelot in a comical misunderstanding.
A mysterious note from Swamp Castle spurs Lancelot to action, prompting an ill-fated assault that leads him to mistakenly slay numerous guests at a wedding, only to discover that the note was not a cry for help but rather a plea from Prince Herbert (Terry Jones), who wishes to escape an unwanted marriage to an unattractive princess.
As fortunes wane, the knights reconvene and bolster their ranks with three new members, including Brother Maynard and his fellow monks. Winter’s touch forces them into drastic measures, resulting in Robin's minstrels becoming their unfortunate meal. They later encounter Tim the Enchanter, who points them to a cave rumored to hold the secret of the Grail's location, guarded by the fearsome Rabbit of Caerbannog. Underestimating the beast, the knights foolishly charge, leading to tragic results as it swiftly dispatches Sirs Bors, Gawain, and Ector. Utilizing the "Holy Hand Grenade of Antioch," bestowed by Brother Maynard, Arthur triumphantly vanquishes the creature.
Inside the cave, they discover an inscription from Joseph of Arimathea, hinting at Castle Aarrgh in France. However, their journey is interrupted when an unexpected animated cave monster devours Brother Maynard, but fortunately, the knights escape just as the animator has a heart attack.
Upon reaching the Bridge of Death, the bridge-keeper challenges them to answer three questions; only those who succeed can pass. Lancelot navigates the queries with ease, but Robin is tossed into the Gorge of Eternal Peril after failing a question about Assyrian capitals. Likewise, Galahad struggles too and joins Robin in a tragic plunge. When Arthur is confronted with an obscure question about swallows, his sharp wit prevails: he asks for clarification regarding which type of swallow is being referenced, leaving the bridge-keeper at a loss and ultimately flinging him into the gorge.
While Arthur and Bedevere seek Lancelot, unaware he has been detained by the police investigating the historian's demise, they reach Castle Aarrgh, only to encounter the French soldiers again. Defeated by a barrage of manure, they muster an army of knights to lay siege to the castle, but just as they charge, the police intervene, apprehending Arthur and Bedevere while shattering the camera, effectively bringing the chaotic splendor of their quest to a close.
