This is an extremely clever, moving and well acted film, and a good antidote to the type of film in which changing the past is made to appear relatively straightforward. But it packs a major sting in the tail and is not one to watch if you want to be cheered up.
The title "The Butterfly effect" is a reference to the behaviour of sensitive systems under chaos theory, and the film starts with the famous quote about how the flap of a butterfly's wings can cause a tornado on the other side of the world six months later. The film could almost have been subtitled "Or: The Law of Unintended Consequences."
The central character is Evan, a boy whose father and grandfather died in mental hospitals and who appears to have inherited a strange condition from them. Evan is portrayed brilliantly as a young adult by Ashton Kutcher, as a 7-year old by Logan Lerman, and as a teenager by John Patrick Amedori.
As a boy Evan has blackouts at times of extreme stress when he does things which can be very strange and scary, and then has no memory of them. To try to help with this, his mother (Melora Walters) encourages him to keep a daily journal.
Evan grows up to become a brilliant student, and then discovers than by reading his diaries and concentrating he can send his consciousness, including his adult memories, back to the time he was reading about. Then he realises that he can try to change the past.
After the suicide of his childhood friend Kayleigh (played as an adult by Amy Smart) Evan sends his mind back to a traumatic childhood event, which he correctly identifies as the start of the process which put Kayleigh on the path to despair and premature death. At first it appears that he has succeeded in transforming her life for the better - but then a side effect of his actions causes something else to go horribly wrong.
Each time Evan tries to go back to undo either one of the original life-wrecking mistakes which he or someone else had made, or the harmful results of his previous meddling, the butterfly effect - and the law of unintended consequences - means that a fresh disaster occurs. Evan does sometimes succeed in saving someone's life or happiness, but only at the price of devastating repercussions for another person.
All three actors who play Evan bring him to life brilliantly, and the acting of the rest of the cast is also excellent. Direction, action and special events are first rate and the pace works very well. There is inevitably a lot of repetition of certain key events in Evan's life as he goes through them again and again, but it doesn't make the film boring because you're looking out for the changes or the explanation. And this film is much more plausible and grown-up than most time-travel movies. But it really does deliver a kick at the end.
There was a "sequel" made a couple of years later called "The Butterfly effect 2" with a different cast of characters, and in which the central character has a similar condition to Evan. However, everyone I know who has watched them both says that the sequel is not nearly as good.
Some great extras on the disc, particularly to sequences in which academics and film makers are interviewed about Chaos theory and about why we are fascinated by time travel stories.
I can strongly recommend this as long as you're not looking for something sweet and cheerful.