Peter Jackson's black comedy, "Heavenly Creatures," is a dizzying pastel debacle of school-girl passion, wild imagination and murderous ambition. The film version of the true story is set in mid-1950s Christchurch, New Zealand. Is it coincidental that "Picnic at Hanging Rock," another tale of school-girl mayhem, is set in nearby Australia? In any case, historical fact is wrapped up in Jackson's fictional version like grotesque reality in frilly paper--this film is a deliciously unpleasant surprise package.

The plot is simple, shocking in the way it unfolds in macabre logic. Fourteen-year-olds Pauline (Melanie Lynsky) and Juliet (Kate Winslet) meet, become friends and grow extremely attached to each other, so attached, in fact, that their parents worry the relationship is "unhealthy" and determine to separate them. In desperation, the girls decide to prevent a separation by any means possible.

Jackson uses Pauline's diaries to conture up the patchwork of reality and fantasy worlds this plot inhabits. The viewer is bombarded by a succession of distorted scenarios--a grainy old film-strip of Christchurch, the Claymation-type animation of Pauline and Juliet's invented alter egos, the "Fourth World" they imagine as their private paradise and the dubious authenticity of mid-century New Zealand. Which world are we supposed to believe in? The extremely exaggerated reaction of Pauline and Juliet to any situation seems to fit the various levels of fantasy existence better than it fits the cramped confines of their mundane homes. And yet, it is when real life begins to demand a fantastic reaction that the film really comes together.

From the very beginning, Pauline and Juliet's intensity is exhausting and marginally believable. One could call it sustained hysteria, but even that would be a toned-down definition of their emotional states. Are they play-acting? (Well, we know the actresses are, but are they playing characters exaggerating themselves?) The film's conclusion makes it obvious that they're not pretending, but even before that, we believe in them. Their energy is a curious mixture of childlike hyperactivity and a more adult sexual tension. They may begin a game of chasing each other through the forest and end lying in the grass, embracing each other in their knickers. Similarly, their beloved clay figurines, characters in Juliet's novel-in-progress about a royal family, have love affairs and illegitimate children.

In the original murder case, the sexual element of the friendship was played up, and the girls became infamous as lesbian killers. Jackson certainly doesn't tone down the sexual angle in his film version. As shocking as the relationship may have seemed in 1954 New Zealand, it could seem hackneyed in this post-Kinsey day and age. However, in spite of modern omniscience and in spite (or because) of the overwrought treatment it receives in the film, the relationship convinces. Pauline writes in her diary that now she understands "the joy of this thing called sin," and after the bathtub scenes and experiences in "The Fourth World," you may find yourself understanding it, too.

Despite Pauline and Juliet's appearing as interchangeable parts in a matched pair, Lynsky as Pauline and Winslet as Juliet are well-differentiated and developed. Juliet comes from the upper class, and Pauline from the very lower-middle, as their respective accents demonstrate. In temperament, Lynsky is a perfectly sullen Pauline, flat-faced and brooding, while Winslet makes a high-strung Juliet, blonde and impulsive. Pauline's ubiquitous heavy awkwardness is less demanding than Juliet's rapid-fire swings from copious tears to maniacal happiness, but both Lynsky and Winslet manage to make their characters more than caricatures of polar emotion.

The secondary characters, primarily the parents of both girls, are flat eccentrics, reminiscent of Joel and Ethan Coen creations in "Raising Arizona" and "Barton Fink." The pansy college professor, the dissatisfied society wife, and others alternately plague their difficult daughters and provide a fairy-tale-gone-wrong background--Pauline's parents, it turns out, were never married, and Juliet's parents are getting a divorce. Pauline's mother, designated victim, is less easy to write off, and it is precisely her authenticity which makes the eventual crime so terrible.

The lesbian relationship shouldn't shock us, and neither should the inevitable murder, but somehow the viscerality and the emotional reality of violence manage to penetrate more effectively in "Heavenly, Creatures" than in a dozen "Pulp Fictions," Peter Jackson's film may flail from giant butterflies to the prossic rituals of tea-time, but in the end he manages to concentrate all the diffuse energy into pure, gritty horror.