The very knotty and naughty "Bachelorette" sets out to explore the tight and often toxic bonds between BFFs that can upend even the best-laid wedding plans. It is billed as a comedy, but it’s really a lipstick-smeared drunken tragedy. The humor is so caustic you won’t know whether to laugh or cry.

I’d probably go with the tears, because it’s also the latest smackdown in the gal-pals-can-be-just-as-gross-as-guys trend that’s become so tedious. Where "Bridesmaids" showed up last summer with shock value and more than a few moments that were pricelessly funny, "Bachelorette" is far more emotionally strung out.

Based on the stage hit by Leslye Headland, who adapted it for the big screen and directs as well, the darkness was actually promising, the issues very real.

‘BACHELORETTE’ Rated: R Opens today at Consolidated Kahala

The perfectionist maid of honor Regan (Kirsten Dunst) conveniently hasn’t forgotten to pack all her lingering resentments. She’s a beautiful, blond, stylishly thin success while the bride-to-be, a plus-sized Becky (Rebel Wilson), has bad highlights and whether she can still fit into her wedding dress remains an open question. Politically incorrect and exceedingly cruel fat jokes abound.

Becky’s "imperfections" trigger egocentric angst that drives the insanity to come. How could she end up blissfully happy with fiancé Dale (Hayes MacArthur), not only a decent bloke but handsome to boot? The irritating question for her bridesmaid "well-wishers" soon escalates into full-blown "why her, not us" outrage.

Headland has stacked the deck against everyone in the wedding party. Gena (Lizzy Caplan) is brilliant, artsy and doing just fine except for the booze, drugs, casual sex and serious depression. Katie (Isla Fisher) is a red-haired beauty who has never met a party she didn’t want to be the life of, or a guy she didn’t want to bed.

This crew arrives on the rocks, but now they are about to be shaken, and stirred. The bitter heart of the film is the bachelorette party itself where all their grievances can, and will, be aired. As we’ve come to expect in films like this, the action will be framed by one major disaster after another. Issues with the wedding dress become a running gag, and these are unhappy women, so meaningless sex and massive quantities of drugs and alcohol factor in.

The bachelor’s best men provide a few more complications. Joe (Kyle Bornheimer), a kind of hunky nerd, is still nursing his high school crush on Katie that he hopes the night might change. Clyde (Adam Scott) is reliving the bad breakup he had with Gena senior year. Trevor (James Marsden) is basically just a pretty face, but Clyde and Joe become the major supporting players as Gena and Katie work through their issues — dry cleaning and paramedics are involved.

Becky, the bride, is the easiest of this wild bunch to like. Her basic decency, and Wilson’s relatively understated portrait of her, provides relief, comic and otherwise. Caplan and Dunst’s characters are harder to embrace, with their nasty edge of narcissism lashing out at anyone who attempts to get close. The saddest of the lot is Fisher’s Katie. As the pretty girl who hopes her body will buy the man of her dreams, Katie inspires pity more than anything else.

Even in this dreary, going-nowhere role, Dunst is fascinating. She has always been adept at doing brittle and bruised, though Regan is just a pale version of her tour de force in Lars von Trier’s "Melancholia." Like everyone else in the cast, just when it seems as if she might get something of substance to work with, the film veers back into anger. "Bachelorette" is one giant pity party and those are never fun for long.