Mattsu is a ten-year-old boy living in Matsuyama who shares a passion for local mysteries with his friend Yanma. Their favorite activity involves hunting for the city's seven wonders. Mischievous and sometimes scatterbrained, Mattsu holds a deep affection for his hometown.

When two men arrive seeking seven hidden gems, Mattsu, Yanma, and their older-sister figure Moburi-san are drawn into a conflict over Matsuyama's future. Their adventure leads them through encounters with the mysterious gem seekers and involves flying castles.