It relies a character blackout for everything. That's basically the movie. The problem is that it goes down the logical path... So Evan didn't know what happened after the mailbox because of blackouts, but what happened would have been unavoidable gossip in the town, and even newsworthy. Outside of the basic blackout mysteries, and Ethan's confusion, there's nothing going on. Well theres a trip to a fortune teller, which seems pretty cheesy filler. Later on, and between the teenage drama, the whole plot is supernatural time wasting. Quantum leap meets Dawson Creek lite. And every time he changes the future, this complex butterfly effect, he just so happens to be around the exact same people and he himself is the same. Even his haircut is the same. Same weight. Everything. Even his journals are at hand. Apart from one time, but fear not. SPOILERS His old birth video has the same effect as the shaky letters. Who knew? What are the odds? So yeah, he goes back to when he was a baby in the womb and strangles himself with the umbilical cord. Then he (presumably) sees how life plays out, even though he wouldn't have been there to witness it.
Movies like this should be smart and the mystery should could from a gold script not a constant crutch of the character.