One hell of a headfuck. Leonard (Guy Pearce) has no short-term memory. He has to write himself little notes and gets tattoos to remind himself what his car is, where he's staying and who these people are. He thinks the last thing he remembers is his wife being raped and killed; and the men who did it hit him over the back of the head and caused this memory loss. He is out to avenge John G., the man his notes tell him is responsible for "the incident". But his best buddy, Teddy - full name John Edward Gammell (Joe Pantoliano) - is a John G. Neither Leonard, nor the audience, knows who to trust.
Especially as things are backward. The whole movie plays in reverse, with intercut scenes in black and white in which Leonard explains his condition and all about Sammy Jankis, remember Sammy Jankis. Five minutes or so of film goes forward, then it cuts back to five minutes before that. It starts to seem to make sense when you figure out how the scenes overlap. You know what happens, but you only learn the characters' back stories in small increments. You're as ignorant and malleable as the memory deficient protagonist.
A special hidden feature on the DVD replays the whole film in chronological order, yet it still doesn't quite make sense. There are gaps in the plot too. No wonder it messes up your head! Writer/Director Christopher Nolan did wonders with the premise of his brother's short story, which is also on the DVD. Only really the concept of short-term memory loss is carried over. Dody Dorn's film editing is testament to Eisenstein's age old theory that film meaning is all about montage.
Nugget: well worth multiple viewings (in both directions), but it will never have the same impact as the first time round.