Santiago, her father, an unemployed chef and Pilar, her mother, a successful architect with work to spare, ease her with the illusion that Ratón Pérez will stop by her room that night, take her tooth and replace it with some money.
who lives on a boat anchored at the port along with hundreds of mice who gather the teeth, clean, sculpt and polish them to turn them into shiny round pearls.
These are taken through the city sewers to a jewelry store own by Morientes - an old friend of Pérez that trades them paying the teeth's weight in gold coins.
What should have been a routine job for the mythical mouse won't be tonight since the most heartless and ambitious thugs decide to kidnap Pérez and take control of his boat and his fortune.
This would be the end of the story for everyone except for Lucía who - with the help of her cousin Ramiro - disobeying her parents and not measuring the consequences will try to rescue Pérez unleashing a fascinating adventure that will prove that you don't have to be a kid to believe.