Miss Bianca, a white mouse of great beauty and self-confidence, travels with the ambassador's son to Norway on behalf of the Prisoner's Aid Society in a perilous mission to rescue a poet imprisoned in the dreadful Black Castle.

On Halloween night when Madame Butterfly slips down the drainpipe, hurts her paw, and loses her nose flute, Jenny bravely volunteers to return her friend's beloved flute, even at the risk of being captured by dogs.