Based on Patrick McGrath's novel, "Spider" is a bleakly brilliant exploration of mental illness. Compared with this film, "A Beautiful Mind" looks like a harmless Rorschach blot.

Dennis Cleg (Fiennes), nicknamed Spider by his mom because of his childhood fascination with arachnids and webs, has just been released from the mental institute where he's spent several decades. His new home is a halfway house in a dingy corner of East London run by Mrs. Wilkinson (Lynn Redgrave), a brusque, tight-lipped woman whose sagging breasts threaten to burst the middle button of her blouse (a hint of sexuality, we eventually learn, that Spider doesn't need).

Whether Spider was really ready to leave the asylum is an open question. Scarecrow thin, gazing at the world through blank eyes and speaking in a jumble of guttural grunts and incomprehensible mutters, Spider is like Jack Nicholson at his typewriter in "The Shining." He spends most of his time in his room, huddled over his journal, furiously scribbling indecipherable words that resemble hieroglyphics. When Mrs. Wilkinson discovers Spider is wearing four shirts, a fellow resident points out, "Clothes make the man. The less the man, the more the need for clothes."

It turns out the halfway house is near where Spider grew up. As he wanders the streets, his memories well up and become real -- or real enough. He sees himself as a boy (Bradley Hall), with his pretty, patient mom (Miranda Richardson, terrific) and his lark-about dad (Gabriel Byrne, likewise), who'd rather spend time at the pub than at home. Not only does Spider witness these scenes, he inserts himself into them, hunched over a table at the pub, where his dad flirts with cackling tarts, or crouched on the stairs, while the family eats dinner. He's like Edward Gorey's "Doubtful Guest," morosely hovering in the corners of his own mad memories.

It's soon clear that what Spider remembers may or may not be true. When he watches his mother apply lipstick or swirl in a slip (asking Spider if he thinks his father will like it), it's hard to tell if she is consciously forcing him to acknowledge her sexuality. Is he appalled by her female flesh or enthralled or both? When his father brings a lewd, sluttish blonde home (also Richardson) and says she's the boy's new stepmom, we have no idea if this is fact or fiction, a real incident or some demented internalization of his parents' relationship.

Admittedly, the Freudian-fueled plot is skin deep. But it's presented with such daring intricacy, with such precise distress and nightmarish intelligence that everything in the movie comes to seem terrifyingly portentous. Cronenberg lets his grim tale out in drizzles and dabs. We're immersed in the movie's stark minimalism, dizzied by the long, slow camera glides that recall "The Long Day Closes" in their peeling wallpaper and hushed melancholy.

Cronenberg has made Spider a character out of Samuel Beckett (Fiennes even has the writer's shock of hair), placed him in a Harold Pinter play (with its chokes and pauses and drips of dialogue) and set the whole thing in an airless '50s-kitchen-sink landscape. This no-frills direction drenches us in images almost tactile in their anguish: Spider, sitting alone on a gray day, silhouetted against a desolate canal and mumble-singing "Silent Night" to himself.

Fiennes gives a towering performance, an acute character study that masterfully conveys the immensity of this man's pain, terror and confusion. Breathing in small, shallow gasps, his haunted eyes fixed on some unknowable torment, Spider is a madman trapped -- hiding? -- in the cobwebs of his mind. Every day, he stops by a drab little cafe where he sits alone and slowly pours sugar in his coffee. Of all the things we see in this movie, this is the most heartbreaking. Does he enjoy this moment? Does the sugar give him pleasure?

That is the wrenching question at the center of this amazing film. Can a mind so lost and hurt and angry find comfort in anything? If his very memories betray him, can the present offer even an instant of happiness? We can only hope so, for the sake of the wretched little Spiders curled inside us all.