Toward the middle of Memento, one of the most involving and perplexing films in recent memory, the lead character is running but he has no idea why. He first assumes he is chasing after someone until he is shot at until he realizes he is the one being chased. Sound confusing?

Get used to it. Memento, written and directed by Christopher Nolan, is about something the average person takes for granted: the ability to make memories. As the story unfolds (in a most unusual way), it becomes apparent just how crucial memory is not just for simple, everyday tasks, but also for something as paramount as our own identity.

The story concentrates on Leonard Shelby (Guy Pearce), a former insurance investigator from San Francisco who is obsessively searching for his wife's rapist and killer.

It's not that simple, though. The man who killed Leonard's wife also hit Leonard on the back of the head while he was attempting to rescue his wife, causing a mental condition known as anterograde amnesia. This condition means that Leonard is unable to transfer his short-term memories into his long-term memory. Everything before his injury he can remember; however, his attention span now lasts a mere five to 15 minutes, after which time every piece of newly acquired information is erased. "Everything fades," Leonard describes.

It's ironic that as the movie continues, while Leonard can only remember the distant past, the viewer can only "remember" the future. This is due to the narrative structure.

Consider this: the very first scene shows a Polaroid of a man (a guy named Teddy who we soon learn more about) shot through the back of the head. The picture fades into black. Blood flowing out of the real man's head all of a sudden reverses its direction. A bullet shell flies back into Leonard's gun and the lifeless body comes back to life.

This magnificently clever opening is literally backward, but the rest of the movie, while told backward, is done so in increments. The second scene goes back in time and ends where the first began, the third goes farther back in time and ends where the second began, and so on.

As Memento goes back in time, we meet the man Leonard eventually kills, Teddy (Joe Pantoliano), a shady tag-along. Like everyone else, it's impossible to know if he's telling the truth.

We also learn about Sammy Jankis, a man Leonard came to know through his work as an insurance investigator with a similar condition to Leonard's. Unlike Sam, Leonard uses a system of tattoos, notes and photos to "remember" important facts.

Another major character, Natalie (Carrie-Anne Moss), is in the tradition of film noir femme fatales. At first, or really toward the end of the story, she seems sincere. As time goes back, however, her motives are less clear.

These characters entangle Leonard in something that detracts him from his goal, which he pursues with admirable determination.

But as Leonard puts it, how can he heal if he can't "feel" time. The memory of his wife's demise is always no more than fifteen minutes old. It's this proximity that drives Leonard.

At a diner one day, Natalie makes an obvious point: Leonard won't be able to remember it when he finally kills the man he is looking for. Leonard's response relates a feeling of objectivity we all hold. He says, "My wife deserves vengeance, it doesn't matter whether I know about it."