"Grease 2" is the most serendipitous sequel in recent memory. It is an ingratiating, jubilant improvement on a crummy original.

And as the box-office smash of four summers ago, "Grease" left plenty of room for improvement, especially as an example of movie musical stylization. The movie's enormous success appeared to be a fluke, attributable to the popularity of John Travolta. Even so, it was depressing to contemplate the fact that "Grease" had become the top-grossing musical in Hollywood history, surpassing even "The Sound of Music" and vaulting into fourth place on the Variety list of all-time hits. The whole tradition seemed to be devalued by the sudden commercial preeminence of this overwhelming esthetic insult to movie musicals.

Evidently, the remarkable, desirable difference between "Grease" and "Grease 2" can be attributed to the promotion of Patricia Birch, who choreographed the original, to director-choreographer of the sequel. One suspects that she must also have cringed at the way her dance routines looked in "Grease," because the first thing that impresses you about "Grease 2" is the pictorial and rhythmic assurance of the dance ensembles.

The movie begins with a complicated outdoor production number, as the student body of Rydell High arrives for a new semester, ostensibly two years after "Grease" concluded. By the time the number ends with a charming acrobatic flourish--a tardy student bypasses the front steps and dives gracefully through an open window--Birch has succeeded in introducing all the significant characters, new and old, while demonstrating a flair for robust, humorous, coordinated dance movement on the screen.

When the brisk, amiable young cast members of "Grease 2" shift into strutting, bouncing, gliding dance steps from their ordinary strides or gestures, the composition and cutting harmonize with the ways and directions in which they're moving. There is a fundamental harmony between the flow of activity and the flow of imagery.

In addition to being energized and controlled by a dancer's vision, "Grease 2" reveals a romantic, sensuous outlook that dignifies much of the sexual joking and maneuvering between the teen-age characters.

Ken Finkleman's scenario stands the plot of "Grease" on its head. The hero, Michael Carrington, played by Maxwell Caulfield, is now the foreign newcomer to Rydell High; indeed, he's identified as a British cousin of the character played by Olivia Newton-John in "Grease." Michael falls for a pseudo-tough cutie, the delectable Michelle Pfeiffer as Stephanie Zinone, a restless adornment of the girls' gang known as the Pink Ladies, whose members are expected to Keep Cool and date only guys belonging to the T-Bones, a harmlessly hoody boys' gang. Seeking a means of impressing Stephanie, Michael transforms himself into the embodiment of her fantasy dreamboat, a mysterious and erotically irresistible Lone Biker.

Birch and Finkleman have altered the tone of the high school mating games that seemed deliberately coarse in "Grease." The approach remains humorous, but it's no longer crass and smutty. The situations may look familiar, but the result is a more amiable form of romantic comedy play. It's also a sexier form, because the change of tone tends to dignify the romantic attractions and attachments, suggesting that the Pink Ladies are something more endearing than teen-age sluts and their boyfriends more endearing than teen-age slobs.

The change proves especially flattering to the female characters. The girls in this blithe, satirical exaggeration of high school mores in the early '60s enjoy parity with the boys and they express an emotional integrity that their counterparts in "Grease" could only envy. At the same time, they exert more erotic force. The romantic longings that seemed synthetic when expressed in song or expression by Olivia Newton-John acquire a smoldering, savory conviction in Michelle Pfeiffer, who seems to combine suggestions of an embryonic Deborah Harry with a bouncy, giddy sweetness.

Although the production seems to be bursting its seams with energy and young talent, not all of it is effectively utilized. Birch and Finkleman come close to turning Rydell High into a juvenile approximation of Damon Runyon, but they lack the quality of songwriting that a Frank Loesser could bring to a show like "Guys and Dolls." You frequently feel that very talented people are doing their utmost to transform essentially second-rate musical comedy material.

Caulfield is a weak spot, although it appears to be the role more than the performance that's at fault. Evidently, Michael was envisioned as some sort of droll combination of Elvis, Ricky Nelson and Troy Donahue at their sleepy-dreamiest, but the joke, if that's what it was, doesn't pay off. Michael just seems too passive for the surrounding show. As a result, Caulfield fades into the background as one's interest shifts to the young actors who get to play raffish, funny kids--Adrian Zmed as Johnny, the leader of the T-Bones, and Peter Frechette, Christopher McDonald and Leif Green as his sidekicks, plus Matt Lattanzi, an authentically funny dreamboat in the fleeting role of Brad, the centerpiece of a high school trio called the Preptones.

Pfeiffer's most prominent and effective sidekicks in the Pink Ladies are Lorna Luft as Paulette, who emulates Marilyn Monroe, and Maureen Teefy as the peppery Sharon, who shares an outrageously promising duet with Frechette--a satirical rendezvous in a fallout shelter--that hovers on the edge of something special before losing its nerve and collapsing like a deflated balloon. Still, it doesn't collapse for lack of effort on the part of the performers.

Indeed, "Grease 2" recommends itself as a sparkling showcase for several fresh musical comedy performers--and a director eager to put a fresh, attractive imprint on movie musicals.