Leia the Little Mouse follows a small, determined mouse called Leia who loves to draw and paint more than anything else in the world. However, the mayor of her hometown of Yoshlish, the oppressive Mews Mursus is determined to stifle the passions and aspirations of its citizens. In this society, people are only allowed to pursue a career dictated by their family business, which is passed down through the generations. Another horrible and restrictive custom is that young people are forced to marry via a lottery. Thus, Leia finds herself promised to an old sewage worker, doomed to a loveless marriage and unable to fulfil her creative passion.
Fortuitously, Leia meets Neil, a young mouse like herself who works as a trader with his dad. Neil’s family is the only one in the city permitted to travel across the sea to Siena, an incredible haven of opportunity where people are free to choose their own destiny. Planning to escape to Siena and become an inventor, Neil encourages Leia to leave Yoshlish with him. After their plan to escape via boat fails, they find themselves on the run from the city police and flee into the nearby dark forest. There they meet Giuseppe, a grouchy, sarcastic chameleon who aspires to be a filmmaker. The travellers cross many lands: the musically inept city of the bats, the gluttonous metropolis of the hamsters and the danger-ridden town of the cats before finally reaching Siena. In spite of reaching their goal, however, life proves not to be as easy as they had thought it would be…