In Thea Stilton #35: Fiesta in Mexico, the Thea Sisters embark on an exciting journey to Mexico City. Invited to join a research team studying monarch butterflies, they immerse themselves in the vibrant culture and natural beauty of the region. Their adventure takes a turn when their friend Luz, an artist inspired by Mexico's rich heritage and the majestic butterflies, faces a crisis: her mural disappears just before a significant art exhibition. Determined to help, the Thea Sisters dive into a mystery that intertwines art, culture, and friendship. As they navigate bustling markets, historic sites, and the Monarch Butterfly Biosphere Reserve, they uncover clues to solve the case and ensure Luz's artwork is showcased as intended.