What happens and why involves some low-grade special effects and a time-traveling conundrum that's been in circulation at least since Ray Bradbury's 1952 story "A Sound of Thunder," in which a man steps on a butterfly in the past and changes the future.
The Butterfly Effect owes a great debt, then, to Ray Bradbury's "A Sound of Thunder," Brian Aldiss' "Poor Little Warrior," William Goldman's Control, and Clive Barker's "The Inhuman Condition"; that it manages to honour to some degree each one of its sources (if only with the precision lavished on the telling of its dank tale) identifies the picture as a most difficult beast to embrace--and just as difficult to dislike.