Grade: D+

The notion of a cheerfully mean-spirited Christmas movie–one that turns the conventions of the sappy holiday fare audiences have been fed for decades on their head–is a great one, and it would be wonderful to report that “Bad Santa” fulfilled its promise. The black comedy by Terry Zwigoff–the director who made both the documentary “Crumb” and the odd, off-kilter coming-of-age tale “Ghost World”–does have a few laughs and an agreeably perverse spirit. But in the end it proves a one-joke conceit that can’t sustain itself over feature length, especially since it’s directed in a clumsy, vaguely junky fashion that might be intended as part of the gag but merely comes off as amateurish (an effect emphasized by the mediocre technical work). And even worse, it goes comparatively soft in the end. (Maybe it does so in an effort to send up the syrupy finales one is accustomed to encounter in the usual Santa Claus stuff, but the effect is still disappointing: it would have been a real act of courage to maintain a bleak, defiantly unsentimental tone to the end.)

The premise of the Glenn Ficarra-John Requa script (reportedly touched up by Joel and Ethan Coen) is very simple. Randy, alcoholic safe-cracker Willie T. Stokes (Billy Bob Thornton) teams up with larcenous dwarf Marcus (Tony Cox) to rob a department store each Christmas. They do so by going to a different city every year, hiring themselves out as a Santa-and-elf combo for the duration. Willie is a terrible St. Nick, treating the kids with disdain, frequently arriving soused and hitting on any reasonably attractive woman within reach, but once on staff his job security is insured by the presence of African-American Marcus, who could easily charge their employers with a double civil-rights violation if they were terminated. The whole point of the operation is that on Christmas eve Tony remains hidden in the store after closing and sneaks Willie back in, so that they can rob the place of goods and cash from the safe and disappear until the following November. But in the year they hit a mall in Phoenix managed by nervous Bob Chipeska (the late John Ritter, to whom the picture is dedicated), Willie–who’s tired of the grind–links up in a crassly ribald romance with a sex-crazed bartender named Sue (Lauren Graham) and reluctantly makes a weirdly paternalistic connection to an oversized, dim-bulb kid named Thurmon Merman (Brett Kelly), who adopts him and, after Stokes has to flee his own pad when it’s searched by the store detective (Bernie Mac), even invites him to move into the house he shares with his addled grandmother (Cloris Leachman).

Where things are going is predictable, but it must be said that in getting there, the writers and Zwigoff insure that what little sweetness is allowed to enter is overwhelmed by a far heavier dose of the bitter. That’s fine. What isn’t is that the level of humor never gets beyond the crudity embodied in Stokes’s very first voice-over diatribe, which consists mostly of a string of vulgarities. And while you might be tempted to chuckle at the first occurrence or two, the endless reiteration soon grows tiresome and tasteless in a peculiarly boring way. Then when Thurmon is introduced, the kid comes on not as strangely charming but so odd that he’s positively creepy. As things progress further, moreover, there are twists–the death of one character, the betrayal of other–that are so grim, and so klutzily staged, that they’re simply mean-spirited, not amusing. (It doesn’t help that the background score consists mostly of curiously inappropriate bits of classical music–the same Shostakovich waltz that Kubrick used in “Eyes Wide Shut,” for example, here gives a carnival-like atmosphere to a murder.) Then there are the unexplained bits of oddball nastiness, as when a psychotic bar patron (Ajay Naidu) attacks the red-costumed Stokes, which are just too unpleasant to elicit more than stunned silence.

The performances are one-note affairs, too. Thornton manages to be convincingly mean and dissolute, but not in an amusingly W.C. Fields way–he’s genuinely obnoxious. (Wouldn’t it have been more plausible, too, if the character were chubbier than a reed?) Cox has a few good moments, but is mostly stilted, and Kelly is persuasively bovine. Mac is pretty much wasted, and though Ritter doesn’t look particularly well, his nervous shtick earns a few smiles. Leachman’s contribution is just another embarrassing cameo.

You can bet your wallet that some critics will praise “Bad Santa” to the skies; bound and determined to approve of any Zwigoff effort, they’ll exult in its supposed edginess and twisted tone, and even say that the technical sloppiness is a virtue. But the unhappy fact is that it represents a good idea drowned in a sea of crudity and ineptitude. You’ll probably leave it feeling more hung over than Willie Stokes.