YOU CAN'T SAY success has spoiled Monty Python--this movie was made before it became the Rowan and Martin of the mid-seventies--but something's done it. Probably just a silly decision to concentrate on a single plot, making the whole hour and a half seem like one extended joke that very quickly loses its savor. Monty Python's best routines have often been its shortest, and the longer ones--like "The Piranha Brothers" and "Fairy Tale"--were very often the only losers on their records. And Now for Something Completely Different was extremely funny, leaping from skit to skit without worrying much about continuity. Monty Python and the Holy Grail, by contrast, has a single unified story line about a deadpan King Arthur searching for, well, the Holy Grail. Instead of being simply a background for a series of more or less independent routines, the Arthurian motif strangles the jokes.

To call the action heavy-handed is an understatement. A knight in the forest bars the way for King Arthur and refuses to admit he is defeated even after Arthur slashes his arm off (blood spurts as if from a faucet); Arthur then chops his other arm off, and then each of his legs, before moving on. There's really nothing funny about it. The keynote of the humor is gore. Even in what may be the funniest moment of the film, when a small white rabbit guarding a cave catapults into the air and saws off a knight's head (more blood) with his teeth, it's too disgusting and unfunny to laugh at much.

The film has almost no verbal humor. A Frenchman shouts insults at the knights, but he's nothing compared to the man in "The Argument Clinic," and the soundtrack compounds the problem by being substandard. Even the music (by ex-Bonzo Dog Band member Neil Innes) is lackluster, without any of the tang or catchiness of "The Lumberjack Song" or "Dennis More."

THERE ARE a few funny reversals of cliches about medieval movies, but these fall into two categories: the "it must have looked funny on the drawing board because it sure doesn't look funny on the screen"; and the simply stupid. In the latter category, the knights build a sort of Trojan Horse to enter a castle and then forget to hide themselves within it. In the former, the defenders of a castle attack the approaching army not with boiling oil or arrows, but with large, living animals like sheep and cows. Even the jokes that are somewhat amusing at first--King Arthur has no horse but his page carries split coconut halves that clop together to sound as if he is galloping--are beaten into the ground by repetition.

It is painful for a Monty Python fan like myself to continue describing the nadir of its enorts. When only a couple of the group's albums were available and the first TV shows went on the American air, it seemed as if the group's standards were pretty high. Now even the dregs are being shown on TV. distributed to movie theaters, put on new records, and even printed in books (incidentally, these books--Monty Python's Big Red Book and Papperbok--are not abysmal but contain only a few new things, and these items are really only likely to amuse diehard admirers). It's clear now that Monty Python is not more consistently clever than any other comedians. If only Time magazine and the other media currently hyping the group would let us, the decent thing to do would be to forget that bloody turkeys like Monty Python and the Holy Grail were ever made.