After a mandate by God himself, King Arthur and his Knights embark on a quest for that holiest of utensils, the Grail. Oh, originally they were journeying to Camelot, but it was deemed to be a silly place, complete with singing and dancing. The quest is met with much peril - a Black Knight who doesn't let a little thing like dismembered extremities get in the way of guarding a pass, a three headed giant, a Bridge of Death (where one must answer certain questions to cross), and a prince in need of rescue from an arranged marriage. Attempts to storm a French occupied (and aren't they rude) castle are disastrous, especially when they attempted to infiltrate via a Trojan Bunny.
Along the way are also accused witches and the Castle Anthrax, to which Sir Galahad followed a Grail-ilke beacon but instead finds a coderie of amorous young ladies. Pythonesque humor often involves death and violence, including a recurring gag where someone is pronounced dead but is actually alive, then summarily finished off. There are also a myriad of puns, anachronisms, inaccurate subtitles, fourth wall breaking, animations (by Terry Gilliam, also co-director of the film with Terry Jones), movie jokes, and great embarrassment for its participants. Of the latter, my favorite bit involves the minstrels surrounding the "brave" Sir Robin, whose songs go into great detail of his hasty retreats from danger.
I see I've given away much of what happens but I couldn't possibly translate the pleasure of watching into words. While quoting lines like "I fart in your general direction" may make my invisible audience smirk, you just have to witness the artistry and timing of John Cleese, Graham Chapman, Eric Idle, and Gilliam and Jones yourself. Maybe you'll not find it all that funny. Maybe some viewers will get through it by noting the curious similarities between HOLY GRAIL and the Mel Brooks comedy BLAZING SADDLES from around the same time.