Beneath our world lies a kingdom of animals.

Woe be to the one who finds the door!

Brother and sister Voltaire and Cerise are exploring their new home in the countryside. Their new playground is simply enormous: there are rivers, trees, and even a few harmless animals… Then, along with their cat Nelson, they discover a strange crack labeled with the name ‘Ringo.’ Curious, they decide to crawl inside, and realize that the crack leads into a passage… but where does the passage end? They pass through a door and find themselves in a room full of heaps of miniature objects! That’s when the lights go out, and Voltaire finds himself caught and tied up by hundreds of mice — who wear clothes and speak just like human beings! Meanwhile, Cerise has managed to escape. She finds herself in a strange underground world, where all the people are animals… Petitmardi, the dog whose job it is to guard the passage, realizes that these ‘yoomins’ are not, in themselves, a danger to the animals. But it’s absolutely imperative that they be returned to their own world, for no yoomins are allowed to live down here. In fact, Petitmardi fears that the menacing ‘Red Ones’ might learn about the yoomins’ appearance, thus triggering an uncontrollable reaction… Naturally, just as in all good stories, Cerise and Voltaire will eventually succeed in re-opening the passageway and returning to their own world… At least, until their next voyage!