A kid finds his dad's old map to Tokyo's underground and decides to actually follow it. What starts as a treasure hunt becomes something stranger, a descent into a world where forgotten people live in the dark, still fighting wars that ended decades ago. The movie treats this collision between a child's innocent curiosity and the weight of history with genuine tenderness, never mocking the old soldier or the eccentrics who've made the tunnels their home. It's adventure that earns its heart the hard way, by refusing to look away from loneliness.

Fifth grader Ryuuhei stumbles onto his father's old journal while packing to move. Its pages record a boyhood hunt for a treasure hidden somewhere in the vast maze of passages under Tokyo. During summer vacation, Ryuuhei forms his own exploration team with friends he met online, and his little brother slips along as they climb down a manhole into the sewers. The kids never expected to find anyone below, yet they meet a whole community of odd but friendly adults living underground, among them a man named Matakichi. The fun takes a turn when the tunnels become a battleground involving an elderly self-styled soldier still carrying out a World War II mission. Can the young explorers find the treasure hidden beneath the city?