Two men awaken in a dark, derelict restroom, their ankles manacled to a short chain locked to a post, a dead man lying out of reach between them. Their only clues to their predicament come from microcassette recordings found in their pockets and a pair of hacksaws too flimsy to cut their chains. To live, says the hollow voice of their phantom jailer, one has to kill the other, or they both will be left to rot.

The opening of "Saw," the ingenious debut of director James Wan and screenwriter/star Leigh Whannel, is almost irresistible. What they wring from it is even more impressive, in a genuinely and profoundly perverted way.

The film unfolds in a series of flashbacks as the two men - a cancer doctor with the bedside manner of a taxidermist (Cary Elwes) and a photographer in a seamy trade (Leigh Whannel) - try to puzzle their way out of their porcelain prison before their allotted time runs out.

The fragments tell the story of "the Jigsaw Killer," a demented genius who "finds ways for his victims to kill themselves," according to one investigator, and the unstable police detective (Danny Glover) who slips over the edge of reason in his obsession to get the mystery killer.

"Saw" is the most extreme entry yet in the bleak and dusky splinter of American thrillers about criminal geniuses with messianic egos and sadistic torments, and Wan and Whannel don't stint on the sadism - physical, emotional or mental.

The filmmakers piece it together with almost clockwork perfection and deliver it with masterful misdirection, creating the most ingenious, eccentric and brazenly jaundiced psycho-thriller to come along in years.

TITLE: "Saw"DIRECTOR: James WanCAST: Cary Elwes, Leigh Whannel, Danny Glover, Monica PotterRATED: R (strong grisly violence and language)GRADE: * * * ½ (on a scale of 5)