It's about this guy who finds out that his girlfriend of two...

It's about this guy who finds out that his girlfriend of two years has had this surgical procedure which has erased him from her memory. So he's freaked out and trying to live with it and he can't, so he decides to have the same procedure. Most of the movie takes place in his brain as she's being erased, and you see their whole relationship, moment-by-moment, backwards from this sort of bad end to the better beginning. Halfway through, as the memories start getting better, he decides he doesn't want the procedure.