Lucía is a very restless girl who is always inventing games and new adventures. Santiago, her father, is a chef looking for a job, and Pilar, her mother, is a successful architect with little time to look after her daughter. When Lucia looses her first tooth, her parents assure her that the Tooth Mouse will take it and leave a coin under her pillow that same night.

However, when the little girl wakes up and looks under her pillow, she doesn’t find any coin, which worries her and feels that something bad has happened to the Mouse. Distressed, Lucia tells her parents what her suspects are, but he doesn’t pay much attention to her. That’s why she decides to look for the Mouse with her cousin Ramiro.

In their attempt to rescue the Tooth Mouse, Lucía and Ramiro meet Morientes, a nice jeweller who gives the Mouse coins in exchange of every single tooth he picks up every night. They will also come across Pipo, Morientes’s nephew, who has no interest in the Mouse getting lost. What seemed to be an easy search at the beginning, becomes a fascinating adventure with an end that will prove that it is not necessary to be a child to believe in something.