Celestine is a young mouse who lives in the underground world of rodents. At the orphanage where she lives, the caretaker known as the Gray One tells scary stories about the evil nature of the bears that live in the outside world, though Celestine doubts they are entirely true. She loves to draw but must soon study dentistry, since that's what all rodents do, and to prepare, she must travel above ground to collect bear cubs' lost teeth from underneath pillows. On one such occasion, the cub's family catches Celestine in the act and chases her into a trash can where she is trapped and spends the night. The next morning, a destitute and starving bear named Ernest discovers Celestine and attempts to eat her. Celestine convinces him to let her go by helping him break into the basement of a candy shop, where he can eat his fill. He is soon caught by the store's owner, however, and arrested. Celestine, who is behind on her quota of collected teeth, agrees to free him from the police wagon if he will help her break into and rob the teeth from the office of the store owner's wife, who happens to be a dentist.