You wake up one morning next to a warm body, but you don't know how she got there, let alone her name. It happens, at least it does in the movies -- even ones as perversely original as "Memento," a time-scrambling film noir about forgetting to remember. The tale of a sleuth with short-term memory loss, it is the perfect parable for the age of Alzheimer's, gingko biloba, and Palm Pilot Panic.

British newcomer Christopher Nolan tells the story backward, then doubles back again so the audience struggles with the clues along with the hero. The simple construct also gives people some sense of life from the hero's frightening perspective -- but it's meant to overload their circuits, too.

Nolan's theory: Our memories are not to be trusted. They're merely temporal souvenirs. Akira Kurosawa toyed with the same notion in "Rashomon," the classic examination of shifting realities and self-serving motives. In Nolan's movie, of course, the motive is the true mystery and the very last thing we learn.

Based on a short story by Nolan's brother, the movie centers on Leonard Shelby (Guy Pearce), an insurance investigator who wakes up every morning and reaches for his wife, but she's not there. Then it hits him all over again just the way it always does: His wife was raped and murdered. . . . He doesn't know how long ago. . . . And he must track down her killer, the man who cracked his skull and left him unable to "feel time."

The motel room where Leonard lives is littered with notes scribbled on napkins and bar coasters and Polaroid pictures, each with a cryptic caption like "Teddy: Don't believe his lies." He looks into the mirror and finds his body covered with tattoos, each of which represents a clue that he managed to write down before his memory faded. He can't know that the motel clerk (Mark Boone Jr.) keeps moving his stuff to smaller rooms, while charging him the same rate.

He repeatedly reintroduces himself and explains his "condition" to the clerk as well as two recurring characters who seem to be on his side: a femme fatale (Carrie-Anne Moss) and the Joisey sleazoid Teddy (Joe Pantoliano). Although short-term memory loss is not conducive to making friends, it can be comical. In one scene, Leonard is running through a trailer park when he suddenly forgets why. He sees a guy running and figures he must be chasing him, but it's the other way around.

The film's haunting subplot, helpfully shot in black-and-white, revolves around Leonard's work for the insurance company. He's almost as preoccupied with this memory as he is with the loss of his wife. It deals with a policy holder with short-term memory loss. The claim was denied after Leonard accused the man of fraud and, well, just maybe Leonard deserves what he got.

The movie is unforgettable, especially in Pearce's startling performance. The actor, who portrayed Russell Crowe's crony in "L.A. Confidential," puts forth the bruised intensity that Brad Pitt brought to "Fight Club." And come to think of it, that film also dealt with the modern phenomenon of not knowing who or where you are in this brave new world of Ramada Inns, Pottery Barns, Borders, Gaps and Starbucks.

"Memento" isn't an easy film. In fact, it might be the perfect date movie for members of Mensa. It does, however, challenge all viewers and gives them plenty to ponder after the credits roll, the lights go out and they reach the parking lot. If memory serves . . .

Memento (115 minutes, at area theaters) is rated R for violence, language and drugs. Guy Pearce plays Leonard Shelby, a man out to find his wife's killer, despite his inability to remember anything for very long.Carrie-Anne Moss plays a femme fatale who seems to side with the hero in "Memento."