The Butterfly Effect
In this interesting and unique version of time-travel, Kutcher's character is able to use his journals to go back in time to a blacked-out episode. Once "inside" the episode, he can use his present knowledge to alter what happened in the past, thus changing the future. After doing this for the first time, he awakens to find himself in bed with his boyhood girlfriend, the same friend that had committed suicide. His college circumstances are quite different, and he seems ill-at-ease with some of them, but he is clearly happy to have his girlfriend alive and in love with him. Unfortunately, tragedy strikes when his girlfriend's deranged brother attacks him, jealous of his relationship with his sister. This sets in motion a series of events in which he decides that he must continually re-enter the past, in repeated efforts to change circumstances. However, each time that he enters the past, he wakes-up in his newly realized present to discover that the changes that he made always seem to have an extreme negative effect upon somebody. The lesson seems to be a loud and clear, "don't mess with the past."
The title of the movie comes from the nickname of Chaos Theory, as articulated by Ian Stewart:
The flapping of a single butterfly's wing today produces a tiny change in the state of the atmosphere. Over a period of time, what the atmosphere actually does diverges from what it would have done. So, in a month's time, a tornado that would have devastated the Indonesian coast doesn't happen. Or maybe one that wasn't going to happen, does. (Ian Stewart, Does God Play Dice? The Mathematics of Chaos, pg. 141)
My interpretation of this theory is that any action we take, however minute, will forever significantly alter the course of the future. For example, I just leaned back in my chair. Had I not performed this action, I might have won the lottery tomorrow. It's heady stuff, but extremely fascinating.
In the movie, Kutcher's character significantly alters the course of all future events, even those seemingly unrelated, when he changes even the conversations that he had in the past. The result is, although he thinks he is performing an action that will lead to a better future than the one he has experienced, he disturbs the course of events in a way that is unpredictable. Thus, "chaos" ensues.