Leonard has suddenly become an Everyman in a potentially infinite purgatory, blindly trying to revenge an act that has already been avenged, and finding himself manipulated, over and over, by people who would use a splendidly configured avenger for their own ends.
Confusing? Absolutely. Demanding? For sure. This is one movie that actually forces to keep re-examining the situation and straining to make mental links. But, dramatically, the device works. Because it puts us in Leonard's shoes. Like him, we don't know what preceded the scene we are currently in.
The story is divided into two sections (one in color, the other in black and white) that alternate throughout the narrative. The color portion depicts Leonard's investigation in roughly five minute segments that are in reverse chronological order. Each segment begins as Leonard's memory has just "reset", leaving him (as well as the audience) unaware of where he is or what he was doing; the segment ends just after the events fade from his memory. The black and white sections are told in chronological order and show Leonard in a motel room conversing with an anonymous phone caller. By the film's end, the two narratives converge into a single color sequence which becomes the climax.
It turns out that this is a substantial oversimplification of the movie's structure -- and that's just one of the surprises that unfolds once you look at the film closely. Some have found the film daunting, and some critics panned it. They're entitled to their opinion, but many of the negative reviews make it plain that the critics didn't quite grasp what Nolan was doing. It's heartening, however, that most critics at the country's major papers understood that the film has immense thought behind it, both technically and thematically.