It's about String theory and stuff! Nemo, the protagonist, is aware of every outcome of every decision he'll ever make. The story is told through flashbacks from an interview with Nemo as he's the oldest man on Earth, and he tells fragments of the different lives he has lived, all products of his choices in life. The first choice we get to see being whether to live with his mother or father upon divorce. Here the story splints into two lives: One with his mother and one with his father. Continues like this for a bit, splitting into more and more decisions and possible realities.
But it's very interesting how he's shaped and molded by decision and environment. I want to upload one clip in particular from that movie
Oh, and it has the best music ever