The film takes place between the 20th and 21st century, and follows Jared Leto's character (Nemo Nobody). It's one of the best depictions of String Theory in film form, showing a number of different ways that ones life is affected by every choice that one makes.
This is a must see film, but is also a love or hate film. I fell in love with it and felt in touch with a great number of the characters in it, and felt for Nemo through all of the painful choices and situations he was brought through.
It's hard for me to think of a film that is similar to this one, but if I had to choose one it would be The Fountain. Both films keep you guessing and have a strong surreal feeling to them. This film keeps you thinking well after you've finished watching it, and will remain one of my favorite films for a long time.