Bachelorette is a total mess and I mean that as a compliment. It’s in the hysterical, drug-laced vein of “Pineapple Express,” in which none-too-bright, self-obsessed characters not only dig themselves deeper into a hole but are too intoxicated to notice their own descent.

Advertised as a darker take on the girls-can-be-gross-too humor showcased in last year’s hit “Bridesmaids,” “Bachelorette,” which was directed and adapted by Leslye Headland from her 2007 play, is too willfully scatterbrained and inconsequential to register as cruel. At the outset, the movie’s three protagonists—bitter, type-A ringleader Regan (Kirsten Dunst), coke-snorting sourpuss Gena (Lizzy Caplan), and coke-snorting airhead Katie (Isla Fisher)—come off as “Mean Girl” stereotypes. As they start grousing incredulously about their overweight, soon-to-be-betrothed friend Becky (Rebel Wilson), bemoaning her unexpected good fortune, you think the movie is going to be mean-spirited in a boring, forced way, a la Cameron Diaz’s tart-tongued turn in the lame “Bad Teacher” (read Sam’s review of “Bad Teacher”). And Becky (whom the other girls secretly call Pigface behind her back) seems too sweet, too guileless, to have ever befriended these foul-mouthed trollops.

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But then you realize that Headland isn’t going for a realistic take on the wedding preparation-obsessed culture of today, or even, particularly, on female jealousy. From the moment the three girls accidentally rip Becky’s wedding dress, “Bachelorette” becomes a dopey, dizzyingly-paced race against the clock, as the girls botch attempt after attempt to repair it (the incongruously lilting classical music score only works to enhance the dizziness). Having set up the characters’ bare essentials–they all abuse drugs, they all go for the wrong men, two of them have an acid wit while the third (Fisher) admits to “never understanding what people say”–the movie sends them into a glorious miasma of bad decisions.

Not three minutes after stating that she’s “done enough coke to be dead,” Gena plunges her nose into a Baby Powder container of it, in the middle of a pharmacy. High as kites, the girls decide to seek help from the groomsmen–including Gena’s one-time high school beau (Adam Scott)–who are attending a strip club, but the more immediate concerns of flirting and scoring more drugs soon take precedence. As the night wears on, Regan’s bathroom tryst with the designated alpha male douchebag of the group (James Marsden) and Fisher’s woebegone swimming pool romance with a sensitive malcontent (Kyle Bornheimer) prove further distractions to the mission at hand.

“Bachelorette” is one of those rare, A.D.D.-afflicted comedies that succeed because of their flaws. The action is dippy and fleeting, and you may question why this quartet of groomsmen was ever friends, why the other bridesmaids let Regan curse and scream at them, why would-be shocking sequences such as an overdose on Xanax and forced bulimia are dismissed as fast as they arrive. But quickly you let yourself get caught up in the world of the three short-sighted characters, how they’re rendered so volatile–decisive one minute, dazed and self-pitying the next–by their partying ways. The inconsistencies in the plot start to ring true, because the women’s brains are so busily inconsistent. When they register as more clear-headed–if not all that nicer–by film’s end, it’s actually touching (even if the filmmakers overdo the poignancy by stealing the Benny and Joon theme song “I’m Gonna Be” by The Proclaimers).

Fisher, Dunst and Caplan are a terrific trio of comediennes, fabulously unafraid of appearing ugly and hardened, but also capable of expertly- timed slapstick. The scene in which the three girls alternately holler at and plead with a baffled hotel cleaning lady are a hoot. The athletic Dunst has a fine time with Regan’s rigid strut; the excessively short dress she wears seems to choke her entire frame. And no one does morning-after looks of horror quite as winningly as the saucer-eyed, long-faced Caplan.

But the most delightful performer is Fisher. In one scene, she does a spastic little dance with a garbage bag, after temporarily replacing the wedding dress bag; in another, she segues deliriously from an innocuous story about “waking up next to a hamburger” to a giddy tale of her suicide attempt. The way Fisher gradually brings her chirpy character’s veiled depression to the surface is as chilling as it is funny. Though you forget most of the tawdry banter in this whiff of a movie the moment you leave the theater, you remember Fisher’s take on the dark side of a giddy ditz.

Check showtimes and buys tickets for “Bachelorette” here