After his mother's death, the 12-year-old Max, son of pharmacist Sternheim, wishes for a pet. One day a dog appears in Max's life. By his power of persuasion he is allowed to keep this dog and names him "Bello". In the meantime a new tenant, Verena Lichtblau, moves into Sternheim's house, and Max is not happy. But further Sternheim has got a present, that's a special mixture. After he has ruined Edgar's harvest with the mixture, the next accident happened: Bello drinks from the mixture, and so he transforms to a human. With Max's help, "Mr. Bello" continues a human-life. And Mr. Bello returns Max's enjoying of life again. But outside of Sternheim's house Mr. Bello gets in trouble, and there the story begins.