"Memento" is a deceptively innocent title for a very singular new film, not really structured like any other film in recent memory. But who knows? "Memento," Latin for "remember," means a reminder of the past, a reminder that might serve to awaken a memory. It also makes for a particularly suiting and moving title for British writer-director Christopher Nolan's quite remarkable work currently playing across the country. (Nolan adapted the screenplay from a short story by his brother Jonathan that appeared earlier this year in Esquire.)

The very first images unnervingly set the drama in action. A man is holding a just developed Polaroid of a body lying face down, clearly dead. And, as we watch, the Polaroid begins to fade, then goes back into the camera, a bullet goes back into a gun and the victim is alive and talking.

The film slowly, bafflingly keeps moving ever backwards to the beginning where the film comes to an end. The hero of this unlikely film noir -- in a wondrously brilliant performance by Australian actor Guy Pearce -- with an impeccable American accent -- is suffering from a peculiar condition, a genuine medical condition known as anterograde memory loss. A person, like protagonist Leonard, cannot retain any recent memories since the night his wife was brutally raped and murdered and he was struck on the head. Everything prior to that incident, he can recall perfectly -- everything since, slips away maddeningly in minutes. The film undeniably has its thriller aspect. Murders happen. There is talk of drugs. But what involves us foremost is the mystery of Leonard's condition. This is no simple thriller by any means.

Compulsively, he takes Polaroids of everyone to whom he talks, annotating them ("Teddy: Don't believe his lies." and "Natalie: She will help you out of pity."). His whole body is a kind of surrealist roadmap tattooed with various cryptic injunctions, including one written backwards so he can read it in the mirror.). He wakes up in rooms and can't remember why he is there or where he is. He has conversations with people like Teddy (Joe Pantoliano from "The Matrix," currently a bad guy in "The Sopranos"), who keeps telling him, "I know, I know. You already told me that."

Leonard, who remembers he was an insurance investigator -- and a good one -- before he underwent the loss of his memory, appears both intelligent and rational while almost constantly feeling desperate. "Concentrate, concentrate," he berates himself as he frantically scrambles to find a pen to write down some vital piece of information before it fades forever from his consciousness. He knows he used to live in San Francisco but doesn't know why he is driving around in down-at-the-heel sections of sunny Los Angeles. Nor does he know why he's driving a Jaguar and wearing an Armani suit, although eventually we learn by the end, as well as where the two dark scratch marks got on his cheek.

As the film moves forward -- or rather, backward -- in time, we note details like the barmaid Natalie (Carrie-Anne Moss, another "Matrix" alumna) bearing the marks of a brutal beating to a later scene of her arriving home bleeding fresh from the beating.

Most of the film is in color except for certain sequences, as Leonard, for example, recollects a case he handled in his other life. Tattooed on his left forearm are the words: "Remember Sammy Jankis," -- a man who appears to be suffering from a similar case of memory loss. These memory sequences are shot in black and white. Then there are the scenes of Leonard urgently talking on the telephone in an anonymous hotel room -- we never do find out to whom. These scenes are in a soft sienna tone.

Leonard's condition yields its moments of black humor. He hears pounding from a closet in his room. He opens the door. A man is seated on the floor, tied up, duct tape over his mouth. Leonard yanks off the tape. "Who did this to you?" "You did," answers the man. Aghast, Leonard slaps the tape back on and slams the door.

"Memento" keeps you on your mental toes like no other film I can remember seeing. Usually you can just sit back and let the film wash over you. Not with "Memento." You have to keep remembering how each scene locks into the earlier one, getting a little burst of gratification when you see how the pieces of this exceedingly complicated puzzle fit together.

By a strange quirk of coincidence, The Washington Post Health supplement this week (April 10) features a story on "Head Games," all about ways and means of exercising your brain to keep it young and lively, listing a slew of books filled with examples of how to freshen the old brain cells. "Memento" fits perfectly the exact kind of mental work-out intended to keep your brain up to snuff.

A footnote testimonial to the talent of actor Guy Pearce. A few years back he paired off with Russell Crowe as 1930s homicide detectives in "L.A. Confidential." Prior to that, he played equally impressively a leading role in "Priscilla, Queen of the Desert," as a flamingly gay drag queen. Clearly, Australia is the breeding ground for superior acting ability these days.