Fisher, Dunst, and Caplan in "Bachelorette"

Bachelorette’s mere existence is a success story, regardless of the film’s quality. Leslye Headland adapted her Off Broadway play—about a trio of women and their ambivalence about and debauchery during the wedding of a loathed frenemy—for the screen and got a few high-wattage stars on board; the movie, first released online rather than gradually in limited release, has already hit number one on the iTunes rental platform. (It comes out in theaters here September 7.)

And Bachelorette, with its alienation of the audience—this film depicts its characters behaving very, very badly toward one another—is a tough sell. But it justifies its existence, if barely. It’s quite possible that this film owes much to the success of last year’s wedding blockbuster Bridesmaids, but the two could not be more divergent. It may have taken a zeitgeist-y, sexy pitch to get this movie onto the hard drives of iTunes users, but so be it: it’s among the edgiest and most daring American comedies in years, if one that occasionally tips too far into miserabilism.

Kirsten Dunst, who’s made a dandy second career in small and daring roles and who ought never to go back to dumb studio fare, plays the near-protagonist, Regan, the maid of honor. Regan begins the film bragging to Becky (Rebel Wilson) about how she’s been working with young cancer victims, beaming the whole while—this is a completely competent woman, we’re meant to learn, with a heart of obsidian. (They’re getting lunch, and Regan orders a Cobb salad with all the elements of a Cobb salad on the side, pushing the joke into the realm of the obvious, as often happens in this movie.) Becky, the one completely good-hearted member of the ensemble, announces her engagement, which prompts a remarkable comic flicker of revulsion across Regan’s face, before cutting to a three-way call (still?) between Regan and the other two bridesmaids, the trio the story will choose to follow.

The rest of the film takes place on the night before Becky’s wedding, as Katie (Isla Fisher) and Gena (a remarkable Lizzy Caplan) join Regan for a bachelorette party that quickly devolves to include only the three of them. The cocaine comes not in bags but, it would seem, in buckets; that the three girls would end up trying on Becky’s oversized wedding dress (which fits two of the slender bridesmaids), tearing it open in the process, only makes sense here.

Little in the following minutes makes perfect sense, though, and the film has the theatrical-adaptation curse of sticking to a very strict through-line at the expense of spontaneity or realism. Why must our three bachelorettes crash the groom’s bachelor party? Onstage, it’s to create more drama in a limited space, to move on to the next scene. On film, weird and unconvincing explanations are thrust at the viewer, but, frankly, it doesn’t matter: Ms. Dunst and Ms. Caplan carry their characters so well that the audience is willing to go down avenues that otherwise seem inexplicable.

Ms. Dunst is utterly fantastic—after a wildly different performance as a depressive bride in Melancholia—playing a maid of honor sublimating her anger and belief that she cannot be loved. She elevates the film past any memory of the more traditional wedding movies of the past—the horrific Bride Wars and The Wedding Planner, or even the superlative but very sunny Bridesmaids—by creating a character who feels drawn from life, even in all her rage. Ms. Caplan’s character comes less from life and more from filmic cliché: she’s the burnout friend who sleeps with one too many unsuitable men out of a nihilistic impulse emerging from the screenwriter’s chic boredom. But Ms. Caplan’s sheer charisma moves the part out of the realm of the sad best friend. The same cannot be said of Isla Fisher’s performance, though. Her role as an irresponsible dumb-bunny is the thinnest in the movie, and her work in the film has not even a hint of intelligence behind the eyes. I’m fairly certain she mispronounced the name of her workplace, “Club Monaco.” And if it was a joke, it was too nasty to land.

In any case, Ms. Dunst and Ms. Caplan power through the middle part of the movie, a section of sheer contrivance, to arrive, suddenly, at a bravura finale. Perhaps their unjustified detour to a strip club mirrors a bad night spent drunk and high; nothing is meant to make sense, really. Once the three sober up and begin to deal with all the previous night’s damages (that dress is still torn apart), the movie suddenly comes together as more than just a pair of good performances and some classy zingers. There’s genuine tension inherent in the struggle to get Becky’s dress fixed on time.

And that tension wouldn’t exist if the movie were in fact what it proclaims itself to be, from its opening credits of the protagonists’ yearbook photos with slurs scrawled across them to the bridesmaids’ calling the bride “Pig Face” to all those drugs and all that vomit: the bachelorettes are neurotic not because they’re angry messes, as they keep declaring, but because they love Becky and they’re scared for their own futures, all at once. If they can’t get the dress to her on time, what hope is there for them? It’s a question fraught with annoying prejudices—being a bride, or a good bridesmaid, is not at the center of being a woman, as women are told by popular culture—that are nevertheless key to American women’s lives.

By its end, the messy Bachelorette isn’t just a good counterbalance to all those bad wedding movies—or the good one that ends with a Wilson Phillips sing-along. It’s a way for any truly flawed viewer (which is to say any viewer, or at least any New York viewer) to stand vicariously at the center of a nasty, but honestly nasty, set of nuptials.

Bachelorette

Running Time 91 Minutes

Written and Directed by Leslye Headland

Starring Kirsten Dunst, Isla Fisher and Lizzy Caplan

Three out of four stars