Eight year old Lucas would like to know how the Tooth Fairy exchanges coins for teeth without getting caught. He is determined to stay awake all night to find out. Meanwhile, a little mouse falls into a trap set by a dishonest contractor who is desperate to tell the world his secret, that the mouse is the Tooth Fairy. Lucas and his family has to do everything in their power to help the sweet little mouse with its mission, which since time immemorial, is to make children happy...

Lucía, an active little girl, looses a tooth. Santiago, her father, is an out-of-work boss, and her mother, Pilar, is a successful but overworked architect, reassure Lucía, by telling her that the Hairy Tooth Fairy (a mouse named Pérez) will come to her bedroom to collect her tooth, leaving some money in its place. A mouse who had been monitoring the situation tells another mouse, who in turn tells another mouse until finally the news reaches Pérez, The Hairy Tooth Fairy, who lives in a boat in the port along with hundreds of other mice. They receive the teeth that he collects, and then clean, shape and polish them so that they can be transformed into shiny round pearls. These pearls are then carried through the citys sewers until finally they reach the jewellers owned by Morientes, where an old friend exchanges them for their weight in gold.