The young mouse Celestine lives hidden in an underground city inhabited by all kinds of rodents. Although she loves drawing, she must soon begin studying dentistry, the profession all mice one day pursue. In preparation, she is given the task of collecting the fallen teeth of young bears on the surface. When she is caught red-handed by an angry bear family during one of her raids, she just manages to escape into an old rubbish bin, where she spends the whole night in sheer terror. When she is found the next morning by a hungry bear named Ernest, she just manages to stop him from eating her on the spot. The chance encounter develops into an intimate friendship, but not everyone accepts it.