Lucia, an active little girl, looses a tooth. Santiago, her out-of-work father, and her overworked but successful architect mother, Pilar, reassure Lucia, by telling her that, that very night, the Hairy Tooth Fairy (a mouse named Perez) will come to collect her tooth, leaving some money in its place. It just so happens that a mouse who had been monitoring the situation tells another mouse, who in turn tells another mouse, until the news fianlly reaches Perez, The Hairy Tooth Fairy. He lives in a boat in the port along with hundreds of other mice who 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 city's sewers until finally they reach the jewellers owned by Morientes, where an old friend exchanges them for their weight in gold. But what would normally be a routine trip for the legendary rodent takes an unexpected turn for the worse when the most heartless scoundrels kidnap Perez in order to take his boat and his fortune for themselves. Discovering this, Lucia, with the help of her cousin Ramiro, disobeys her parents and sets out to rescue Perez, thus beginning a fascinating adventure with an outcome that just goes to show that you don't have to be a child to believe.