By MARY ELIZABETH WILLIAMS

"Mystery Science Theater 3000" may have the dopiest, cheapest concept since Woody Allen dubbed a Japanese caper with egg salad jokes and called it "What's Up Tiger Lily?" Unburdened with most of the trappings of conventional films (plot, stars, budget), it's a glorified episode of a TV show that took cheap shots at the dregs of celluloid. It's also the funniest movie of the year so far.

The bookending "plot" (and I use the term loosely) will be familiar to fans of the series -- a mad scientist (Trace Beaulieu) wants to enslave the human race, and his instrument of destruction is bad cinema. He shows terrible movies to a hapless test subject who's been shot into space (Mike Nelson), victim watches movies, victim makes catty comments. That's it.

Yet with that simple premise, "MST3000" attracted a devoted following of enthusiasts over the years. The combination of stunningly dreadful features with press-on gags struck a chord of recognition in armchair comedians across the land, folks who already knew the computer isn't the only interactive entertainment device in the house. It was the sedentary person's "Rocky Horror" -- no costumes needed, no time warp, and the yelling at the screen was already done for you.

Director-Producer Jim Mallon and company have wisely opted not to mess with the formula. Mike and his robot pals Tom Servo, Gypsy, and Crow get to wander the Satellite of Love a wee bit more, but mostly what we still see are the backs of their heads in the lower corner of the screen. And mostly what they do is still mutter their amused disgust at the cinematic travesty before them.

The film they're enduring this time around is "This Island Earth," an atomic-age sci-fi "classic" that stinks like sulfur in July. There's some business about cranially overdeveloped aliens, a dying planet that looks like a Yes album cover, and the brilliant yet stupendously naive earthlings whose aid the aliens enlist. But the original story of "This Island" is utterly irrelevant. It's the juxtaposition of cheesy special effects, wooden dialogue, and don't-quit-your-day-job acting with a steady stream of snide remarks from the "MST3000" crew that elevate it, giving it a brilliant lunacy that outshines any dramatic pretensions its creators may ever have entertained.

Freed from the eyes of the FCC, the tone of the movie might be expected to be broader and bawdier than it dared to go on television. But "MST3000" without a giggling delight in its own naughtiness would not be nearly as charming. There's snickering over an alien's panty lines and a scene involving fart humor, but it's all done with the innocence of kids at camp who've just learned a new song their parents wouldn't approve of. A shot of a pilot flying over a snowy mountain range prompts cries of, "Oh jeez, there's soccer teams everywhere!" The closest anything comes to adult humor is when one character utters the rather quaint "whoop-de-shit."

The constant patter from the peanut gallery over the film within the film does not so much produce an Altmanesque tapestry of dialogue as it does an occasional babble problem. Pay attention to the movie and you miss the jokes. Pay attention to the jokes and you'll miss what it is they're joking about. The commentary keeps coming at the rate of the flaming Frisbee that was apparently used as the alien craft in "This Island Earth." But as anyone who's sat through a recent episode of "Saturday Night Live" will attest, there are worse comic crimes than injecting too many bon mots into the action.

"Mystery Science Theatre 3000" doesn't have the top-notch cast or screwball sensibility of a "Flirting with Disaster," or the production values and sophisticated style of a "Sense and Sensibility." And it's probably no funnier than the "Manos: The Hand of Fate" episode of the TV show. So why see it? Because the show is gone, because at its best it was laugh-out-loud hilarious. And this is it at its best.

There is something subversive and exhilarating about dissing what's on TV or up on the screen -- it turns the viewer into an instant rewrite team. And "MST3000", with its everyman hero, works because it pokes fun in exactly the way we've all done. The protagonist is playing the role of us. Perhaps it's that recognition -- we're an audience watching an audience -- that adds to the sense of camaraderie created by the film. We're all in it together, it seems to say, us against stupefyingly lame Hollywood product. The illusion that it's just a bunch of wiseguys making fun of a movie is what prevails. It's a pretty impressive special effect.