In the summer of 2006, fifth-grade student Ryuhei Ozaki discovers a notebook of his father Shohei from his elementary school days titled “Great Tokyo Exploring Journal.” Ryuhei calls together his online chat friends Shun and Yoshio to form a new exploration party. Ryuhei’s younger brother Sasuke decides to tag along to make four. One day during summer vacation, seeking a hidden treasure marked on the map, they descend underground via a manhole.