Carola is a mischievous girl who doesn't care much for school – except for gym class and recess, of course. Without her good friend Willi to keep her on the straight and narrow, she would really be in trouble.

One day at school, Carola invents what she calls "International Ghosts' Day" and a ghost named "Buh" to go with it. When Buh turns out to be less than imaginary, the two decide to switch places, with Buh doing all the schoolwork and Carola taking the opportunity to play practical jokes on all her friends. However, Carola soon finds that being a ghost loses its appeal, and when she decides to switch back, Buh doesn't play along. It's up to her best friend to step in and help her get her body back.