Along comes "Spider," an eensy-weensy movie sustained by two utterly gigantic performances.

Still, the movie is more the vision of its director, David Cronenberg, than of its two mega-talented actors, Ralph Fiennes and Miranda Richardson. Cronenberg, a Canadian madman, made some of the spookiest horror movies in history -- "Rabid" and "The Brood" come to mind -- in the early, less respectable phase of his career. If "The Brood" doesn't creep you out, you've got an imagination clotted with 10W-40.

But Cronenberg was, alas, discovered by the mainstream, got great reviews, blew out of his cult, and soon began to take himself quite seriously. He abandoned horror, even to the point of getting some Hollywood A-list projects, like "M. Butterly" and "Dead Ringers." Of late, he's wandered into a strange zone all his own: He hasn't quite gone respectable, but the films ("Crash" for one, "eXistenZ" for another) now are shorn of the supernatural and instead set in a queasy neighborhood wedged between porn, pathology and ultra-violence. It's a tough place to visit, haunted by specters from the reptile part of the brain, as realized in what might be called the Canadian house style, which is a serene, aestheticized formality.

The end result -- and "Spider" delivers this value with a vengeance -- is a tone that might be called high-church macabre. It chills, it disturbs, it resonates, it gets into your brain. Basically, Cronenberg has somehow trapped a nightmare in a bottle.

As "Spider" has it, the nightmare in question is mental illness. A ruined man, released from an asylum after many years, returns to his neighborhood and feebly attempts to sort out what happened to him -- and what happened to others who ran into him, mainly his mum. Oh, Mum, poor Mum, Dennis has done something naughty and he's feeling so glum.

Dennis Cleg is a man-child, one of the walking wounded from the dark side of the moon. We first meet him as he gets off the train. Cronenberg's clever camera tracks down the length of the conveyance, past hustling and bustling real people with missions and families and lives, until it finally hooks on Dennis, the last man out of the last car, who moves with the tentativeness of a spider missing four legs.

Dennis, with his matting of beard, his seven shirts worn at once, his dirty fingernails, his just-electrocuted hairstyle and his eyeballs dewy with apprehension, is played by the weirdly beautiful Fiennes, who, even more weirdly, seems always drawn to weirdly unbeautiful roles. His Dennis is a mess; he is only marginally functional and radiates irrationality. Some will say: "At last he has a life" and weep a sentimental tear. Others will say, "What is this dangerous fruitcake doing out of his straitjacket?" and load the shotgun under the bed.

Dennis, returned to a halfway house (run by a severe Lynn Redgrave), begins to poke around his old neighborhood. In his clumsy way, he is returning to his childhood; he's investigating his own shattered id, sorting among memories, trying to put 2 and 2 together, frustrated that it always comes out 5.

But he keeps at it, scrawling gibberish in his notebook, muttering darkly in some strange tongue, ignoring the unfortunately too-colorful other patients ("I just got a letter from Sophia Loren," one announces), trying to avoid Lynn Redgrave.

But as he wanders through his old neighborhood, it begins to come out: As a boy, a particularly cold, bland twit (played icily by Bradley Hall), he watched in horror as his father, Bill (Gabriel Byrne), disposed of Dennis's lovely mum and cruelly replaced her with the pub's bawdiest, most brazen, most blond tart, played by the great Richardson. It seems to have driven Dennis to the mad violence of revenge.

That's really the movie, though Cronenberg has found a novel way of dramatizing the past. Dennis doesn't remember; the convention of the flashback is avoided. Instead, he witnesses; he is there in his childhood, an invisible observer, watching as his seemingly victimized younger self writhes in agony at his father's evil, his mum's innocence, the tart's blowsy brassiness.

You can probably guess where this one is going. It's one of those Terrible Secret movies where a last-second revelation lays bare the true horror of what you've just witnessed. Unfortunately, the psychology is so simple-minded that the Terrible Secret (which I've purposely disguised) is quite predictable. The final revelation isn't a jolt so much as a disappointment. Not that, you think. But, yes, that.

The acting, however, goes beyond fabulous. Both Fiennes and Richardson give so much, it's a little unsettling. These two are deep. Fiennes just goes and goes until there's no getting back, and not a twitch of regret or irony or, even, acting anywhere in sight. But it's really Richardson's movie, and the constraints of the too-clever plot require of her a versatility that's almost transcendental. I can't tell you more, but she's a lot busier in this film than the plot synopsis suggests.

But the best thing about the film, I think, is its sense of rot. Though he falls prey to the pathetic fallacy (did it have to be raining when Dennis returned home?), Cronenberg, assisted by cinematographer Peter Suschitzky, sets the movie in the world of Dennis's broken mind: It's greenish and decayed, and everywhere, like too much mold, is the sense of pathology. When it's over, you think: See ya. Wouldn't want to be ya.

Spider (98 minutes, at area theaters) is rated R for violence, nudity and general despair.

Ralph Fiennes as the tormented Spider, a resident in a halfway house for the mentally ill. Fiennes and co-star Miranda Richardson give the film its legs with two huge performances.