Two sisters move to the country with their father in order to be closer to their hospitalized mother, and discover the surrounding trees are inhabited by Totoros, magical spirits of the forest. When the youngest runs away from home, the older sister seeks help from the spirits to find her.

Mei is eating a caramel on one very windy day, when suddenly a whirlwind appears and starts chasing her. The whirlwind turns out to be a baby cat bus. Mei offers the kitten a caramel and the two become friends. That night, the baby cat bus visits Mei. Letting Mei ride inside him, he leaps out into the sky. They head for the midnight forest full of ghosts. Other cat buses large and small have also gathered together.