Leonard Shelby (Pearce) has been suffering since he saw his wife raped and murdered. His affliction, he has no short-term memory. Which means that thirty seconds after something has happened, he has forgotten about it. This means that the last real thing he remembers is his wife being raped and murdered. So naturally, he wants to exact revenge. He has a system set up to help him in his endeavour; he takes photos and notes. He tattoos the important stuff onto his body.
What is really fascinating about Memento is that the plot takes place backwards. In the really cool opening shot, we watch Leonard suck a bullet out of somebody and into his gun, backwards. However, that's the last time an individual scene is in reverse. After that each scene is a jump backwards, so the first scene is the last in time and the last scene is the first in time. By doing this, the audience is put into Leonard's world.
It does sound like the ending is given away at the start and there won't be a climax; however there is a very good ending and a very interesting story the whole way through that left me with one of those funny I-don't-understand-the-world sort of feelings I get when I leave a really good movie. My personal pick for movie of the year.