The movie itself is phenomenal. There are no real bad guys, nor are there any real good guys—just several groups of characters (not all are human) trying to maintain their way of life.
It starts out with Prince Ashitaka defending his village against a demon and getting infected during the fight. The oracle of the village says that he is fated to die and that the demon that attacked the village came from the west. If he is to have the cursed removed (if possible) it is from the west that he must go. So he leaves the village heading west.
Where he comes across Irontown, lead by Lady Eboshi. It's a mining town that is being besieged by animal gods from the forest. She wants to stop the animal gods from attacking the iron works and is willing to go to great lengths to do so.
But aside from that she isn't bad either—the work force is drawn entirely from women slaves and prostitutes and in her inner sanctum she has Leper servants who would otherwise be ignored and left to die on their own.
She inlists the help of Prince Ashitaka and Jigo the Monk, an opportunistic fellow who, if he provides the head of the Forest Spirit, will gain favor with the Emperor.
But—the animal gods in the forest are trying to fight back, helped by San, Princess Mononoke, who was raised by Moro the Wolf goddess as her daughter. In fact, it was a boar god that had turned into a demon through his hatred of humans and had gone rampaging and was the one who wounded Prince Ashitaka, who is fighting the demonic infection and yet uses the power it gives him to keep one side from wiping out the other and manages to gain the trust of both sides.
It's a compelling story and far from a child's tale, despite being animated.