Summer, 2006. A 5th grader Ryuhei discovers a notebook entitled "Tokyo Exploring Records." It was a journal kept by his father, Shohei, when he was around Ryuhei's age. Ryuhei forms his own exploration party with his friends Shun and Yoshio. One summer day, the party and Ryuhei's younger brother Sasuke who happens to tag along, heads to the dungeon looking for a treasure indicated on the journal.