The girls find out that a planned visit by Yasuko has to be postponed because of a setback in her treatment. Satsuki, disappointed and worried, angrily yells at Mei and stomps off. Mei decides to walk to the hospital to bring some fresh corn to her mother.
Mei's disappearance prompts Satsuki and the neighbors to search for her. Eventually, Satsuki returns in desperation to the camphor tree and pleads for Totoro's help. Delighted to be of assistance, he summons the Catbus, which carries her to where the lost Mei sits. Having rescued her, the Catbus then whisks her and Satsuki over the countryside to see their mother in the hospital. The girls perch in a tree outside of the hospital, overhearing a conversation between their parents and discovering that she has been kept in hospital by a minor cold and is otherwise doing well. They secretly leave the ear of corn on the windowsill, where it is discovered by the parents, and return home on the Catbus. When the Catbus departs, it disappears from the girls' sight.
In the end credits, Mei and Satsuki's mother return home, and the sisters play with other children, with Totoro and his friends as unseen observers.
Source: http://en.wikipedia.org/wiki/My_Neighbor_Totoro