In his book The Life Of Reason, philosopher George Santayana comments: "Those who cannot remember the past are condemned to repeat it". Former insurance investigator Leonard Shelby (Guy Pearce) would seem to be a case in point; a man suffering from a rare, untreatable form of memory loss which prevents him remembering what happened 15 minutes ago.

The condition has plagued Leonard ever since he discovered a masked assailant brutally assaulting his beautiful young wife (Jorja Fox), and was knocked unconscious in the ensuing struggle.

Ever since, Leonard has spent every waking minute in the pursuit of vengeance. Tracking down and punishing the man who raped and murdered hisbeloved and left him an emotional cripple. It is a long and tedious process, rebuilding his life from police reports, charts, maps and captioned Polaroid photographs.

The most important details are tattooed across his body, including the initials of the assailant and a car licence plate.

Unable to remember the people he meets, or the conversation he has just had, Leonard is wary in his trust of others. His condition renders him thoroughly ill-equipped to assess either their motives or their basic decency.

However, out of necessity, he must rely on some strangers to achieve his goals, and seeks help from waitress Natalie (Carrie-Anne Moss) and undercover cop Teddy (Joe Pantoliano). For Leonard, the past is a vast blank canvas. The time has come to fill in the missing details.

Memento, writer-director Christopher Nolan's long-awaited follow up to indie hit Following, is a fiendishly clever and ambitious thriller, recounting Leonard's story in alternating parallel time-frames. The first (shot in colour) unfolds in reverse, each scene taking place at an earlier point in time than the one before.

Meanwhile, the second time-frame (shot in black and white) recounts events in the correct chronological order. The two time-frames gradually converge, intersecting in the closing moments of the final reel with a neat twist to completely shatter our preconceptions.

By abandoning the constraints of linear time, Nolan is able to constantly challenge the audience. With most thrillers, the question on our minds is, "What happens next?" With Memento, the question becomes "Why?"

As the film progresses, what becomes most clear is that we know less than we thought. Conversations and events question the validity of Leonard's own story: is he really suffering from memory loss, or is it a clever ruse to mask the compulsions of a psychopath?

Pearce turns in another wonderfully measured performance as the victim struggling to find himself within the rag-tag remnants of his past. He moves effortlessly from moments of utter confusion to blinding realisation. Moss and Pantoliano play on their ambiguity of their characters and leave us guessing as to their true intentions.

Nolan's direction is assured and he juggles the time-frames with consummate ease, constantly questioning the reality of Leonard's universe. He places us in a similar position to his lead character, searching for answers amid a mass of snapshots. When the end credits roll, many questions still remain unresolved, some tantalisingly just out of reach.