The anime is an adaptation of the manga, with the same name.
In order to satisfy their own needs for land and resources, Man is encroaching on their sacred territory, cutting down the trees and destroying everything else that stand in their path. As a result, the animal gods became angry and attacked the encampment.
Into this conflict wanders a young boy, Ashitaka, a refugee of the Emishi people. His people still live in hiding, but when his village was attacked, he shot and killed one of the animal gods. For this he has been given the curse of death.
He has begun a search through Japan to find the secret of his curse when he arrives at the encampment. Outside of the encampment he is attacked by the "Monster Princess", a girl raised in the woods by Moro no Kimi (a 300 year old female dog god) and who is driven by a fierce hatred for the humans who are destroying the forest. She has repeatedly attacked the encampment, but after she meets Ashitaka, she becomes torn between her loyalty to the animal-gods and the world of the humans.
Surrounded by the warring characters of the humans on one side and the animal-gods on the other, each side believing in the righteousness of its cause, it is Ashitaka and San who must face each other personally and, by the end of the film, come to an understanding of each others world and make the attempt to coexist without hatred and violence.
Andou Masashi, Kondou Yoshifumi, Kousaka Kitarou