I guess this is an example of what is called- non - linear narration, a rather interesting technique. The story starts at the chronological end, and works its way backward, taking you through interesting questions about "memory". The main protagonist suffers from a "condition" as he calls it - he remembers things clearly up to the day of a head injury , after that he can not make any new memories, everything fades away.
Good movie. Sort of a whodunit. Keeps you guessing from the beginning and doesn't let you down in the end!