The bare-bones plot is fairly simple: a man named Leonard searches for his wife's murderer with the assistance of a couple of shady characters.
Writer/director Christopher Nolan then adds his first twist: the trauma of the murder has caused Leonard to lose his short-term memory, such that he is now unable to keep in mind what he is doing every few minutes. It also makes it a bitch for him to meet new people. To help himself out, he takes Polaroids of everyone and everything, and then tattoos the really important facts all over his body.
The real kicker is thatNolan tells the story backwards. It sounds like a gimmick, and if the film were weaker, the move would have been laughable or too clever for its own good. But under Nolan's direction, the backwards chronology is not simply used for novelty. Instead, it forces the audience to be, like Leonard, constantly disoriented. By the time you are able to figure out what is happening in any given scene, Nolan takes you back another few minutes and you are thrust into another new scene.
This structure screams out for surprises, and they come along with amiable regularity, culminating in an ending (which is actually the story's temporal start) that itself takes a car ride home to fully process. Thankfully, plot holes are not an issue; Nolan was smart enough to keep the basic story simple enough that a mental review of the plot makes sense.
As for the acting, it all does unequivocal justice to Nolan's movie. Two alums of "The Matrix," Carrie-Anne Moss and Joe Pantoliano, show up to dispense advice to the perpetually unrecognizing Leonard. Again, like him, the audience is unable to fully figure out their motives and characters for much of the movie.
The movie really centers around Guy Pearce, who plays Leonard. He is in every scene of the movie and must constantly convey both his own disorientation and his driving underlying purpose in life. Pearce (the man who wasn't Russell Crowe in "L.A. Confidential") takes the role and inhabits it so fully you couldn't imagine anyone else as Leonard.
At the start of every scene, Leonard loses his understanding of what he is doing, yet he remains the same person throughout. As such, Pearce's acting must simultaneously provide enough continuity that Leonard's essential personality is apparent, while showing the jarring discordance of always being in a new situation that constitutes Leonard's life. That Pearce does so is the major component of the film's success on an emotional level.
Even if "Memento" were told in a straightforward manner, it would still be better than three quarters of the movies currently in release. The backwards structure makes it exponentially better. It is simply an excellent movie.