"The Pirate Movie," a blockheaded travesty that fancies itself a rollicking update of "The Pirates of Penzance," was evidently designed to lure The Young Audience to Gilbert & Sullivan. It was also designed to get a jump on the competition: in this case a presumably straightforward movie version of Joseph Papp's "Pirates" due to open this Christmas.

The supposedly irresistible bait in "The Pirate Movie's" feeble roster includes Kristy McNichol and Christopher Atkins, the nature boy from "The Blue Lagoon," in the roles of Mabel and Frederic, a profusion of anachronistic or tittery jokes and a score that imposes slurpy pop ballads on selections from the Gilbert & Sullivan score.

These ingredients are served up in a spirit of relentless, doddering facetiousness by director Ken Annakin. Stilted where he intends to be spontaneous, Annakin sustains that peculiarly amateurish form of movie illusion in which each shift of camera angle mysteriously reminds you of the drudgery of the filmmaking process. Ah, yes!, you realize, it could have been hours or days--maybe even weeks--between that klunky camera setup and the one that follows it. Funny how a fluid directing style tends to obscure this perception of workaday tedium.

Before subjecting Gilbert's libretto to jocular violation, screenwriter Trevor Farrant is obliged to contrive a story set in The Modern Period. This requirement is met by introducing Kristy McNichol as an adolescent caterpillar named Mabel who inexplicably trails along after a gang of flighty, shapely girls, serving as the group flunky and laughingstock. On a dockside outing she's attracted to Atkins, a dimply juvenile who happens to be staging dueling exhibitions on an old ship with a middle-aged partner, Ted Hamilton, apparently a second-rate actor in need of a gig.

Mabel turns into a boldly romantic butterfly in her fantasies, which are triggered when she's shipwrecked and washed ashore. From the sodden, slumbering Mabel we switch to "The Pirates of Penzance." The nerdy contemporary heroine, oddly reminiscent of the young Jerry Lewis, reappears as a spunky operetta heroine. The dueling young dreamboat who caught her eye is now Frederic, the ingenuous apprentice pirate, and the second-rate actor is his mentor, the Pirate King. The girl gang that tormented Mabel is transformed into a throng of fluttery, submissive older sisters, residing at the country estate of their papa, a bluff major general.

While retaining the outline of Gilbert's plot and the characters and songs that suit them, the filmmakers take incessant liberties. Where the pirate chorus might once have exclaimed, "Oh, dash it all," the rogues now repeatedly shout "Oh, s---!" in sidesplitting unison.

One or more of the lyricists has even been encouraged to deface the original songs with additional verses, usually smutty. Keelhauling would be too good for them. Judging from the swoony lyrics they entrust to Mabel and Frederic in the "modern" songs--"Hold on, nothin's gonna stop us if we just believe in love," for example--this is not a song-writing gang that ought to be humored.

There is one comic element deployed with a semblance of skill--the chorus of bobbies, who march into camera range with a funny close-order precision while singing "Tarantara! Tarantara!," as if they were entered in an event in the Monty Python Silly Olympics. The other sources of legitimate humor tend to be inadvertent and embarrassing.

Ted Hamilton's Pirate King, for example, is a curiously camped-up monstrosity--sort of a short, muscle-bound Randolph Scott crowned with a platinum, early Grace Kelly hairdo. And if Christopher Atkins was flirting with stardom, at least among teenyboppers, after "The Blue Lagoon," he'll be lucky to come out of "The Pirate Movie" with the chance of becoming a stand-in for Willie Aames. It's painfully apparent that he can't read lines, modulate his reactions or generally protect himself from looking an utter ingenuous imbecile.

Atkins' first drippy song number stirred a strange restlessness among spectators--people began filing out of the auditorium and chatting out loud to avoid sharing in the humiliation of it all. Under the circumstances it really is cruel of the filmmakers to give Mabel a wisecracking aside in which she refers to poor Frederic as "My love, the wimp." Christopher Atkins has probably been wimped into obscurity.