Playing a character who's a little stupid, without patronizing him, is one of the hardest things for a smart actor to do, and part of what makes Bob Hoskins' work in "Mona Lisa" so terrific is that he pulls it off. Hard and squat, Hoskins moves through "Mona Lisa" like a pinball -- an innocent in a corrupt world, he's moved by forces he only dimly comprehends.

Having just finished a seven-year prison term, George (Hoskins) goes to his old haunts looking for work. So he's fixed up with a high-class prostitute, Simone (Cathy Tyson), not as a pimp, exactly, but as a driver. In his old lump of a white Jaguar, he takes her to swanky hotels and the home of a wealthy Arab (whose butler serves George tea in the driveway). He also takes her to King's Row, a nightmarish dispatching place for low-class prostitutes.

Simone goes back to King's Row to remember the hell she escaped from, the days when the front seat of a car was her office. But she's also there to look for a girl named Cathy. Desperate to find her, she enlists George's help; by this point, he's so in love with her, he'll do anything, and Simone is all too willing to take advantage of that love. What follows are double-crosses and double-double-crosses on the part of Simone and a snooty pimp named Mortwell (Michael Caine), with George perpetually the one being crossed.

Written and directed by Neil Jordan, "Mona Lisa" gets some mileage early on out of the chemistry between Hoskins and Tyson, a tall, sleek beauty with an ease before the camera that can erupt into a frightening intensity. And there are a couple of scenes that have the messy power of real emotion, as the pair struggle with, and stumble over, George's simple, dumb love for her.

Hoskins' George has the peculiar power of concentration that a dense guy has, and that's the subtext of everything Hoskins does. If he's driving, he's utterly focused on the road, and if he's meeting with Mortwell to get something out of him, he won't be distracted from that -- he hangs onto his objective like a dog with a bone. What he clings to, throughout, is an idea of decency; however corrupt his world, he can't let go of that, and it becomes his tragic flaw.

But "Mona Lisa" isn't a tragedy -- it ultimately fails as a kind of fuzzy film noir. Instead of the kind of intricate plotting the genre requires, Jordan throws in a lot of caper stuff, chase scenes and the like; where doom should come as inevitably and decisively as nightfall, "Mona Lisa" sputters to its conclusion.

The best film noirs were made by corrosive wits like Billy Wilder, but Jordan's heart isn't in it -- he identifies with the dumb loser instead of the femme fatale. "Mona Lisa" is consistently undercut by sentiment, whether it's the cute routines between George and his best friend, a mechanic and junkman, or the "heartwarming" stuff between George and his estranged daughter. In the end, "Mona Lisa" is another movie about the lovable little people; the movie is mushy where it should be monstrous.

Mona Lisa, opening today at the Circle West End and Circle Avalon, is rated R, and contains graphic violence, nudity, sexual situations and profanity.