Millie, an orphaned mouse, is being raised by a family of hedgehogs who love her dearly. However, she is afraid of everything, especially the crocodile who bit off her tail when she was a baby. Her closest friend is Morten, the son in the family. One day, Millie accidentally kicks their football into the river, and Morten tries to retrieve it but falls into the rushing water and is carried away. Millie is terrified, but she knows she has to find Morten. She embarks on a journey to find him, accompanied by the Frog Archimedes, who agrees to help her. As they journey together down the river, Millie struggles to find the courage she needs to search for Morten. With the help of an ancient turtle, Shelley, who possesses a magical courage stone, Millie finds the strength to face her fears.