The little orphan mouse Célestine lives in the underground world of rodents. At the orphanage she learns to collect teeth from the scary world of big bears. During one of her tooth hunts she is nearly eaten by Ernest. An unlikely friendship eventually arises between the two, but being a friend of a bear does not prove to be completely trouble free.

Going to pick blackberries, Celestine discovers an old mill invaded by ivy. Ernest claims that the building is haunted, but instead of scaring Celestine, it perks his curiosity ...

Ernest has lost his accordion's button and Celestine is convinced that it is the Green Mouse, a mysterious rodent that recovers lost items, has it ...

Ernest is tired of being overseen by his neighbours. He has an idea of how he, along with Célestine and Bolero, can do something about it.

Ernest is sulking because his bees have begun producing bitter inedible honey. Célestine is in good humour since Madam Tulip's flowers are lovelier than ever.

The mice prepare for their annual ball and Célestine receives her first invitation. Ernest is glad, but the other bears cubs have a negative view of the festivities.

Augustine boasts that he saw the famous monster running near the lake. Célestine and her friends ask Ernest to take them there immediately!

Ernest becomes a museum curator to allow Celestine to paint. When a painting is stolen, he is suspected!