Ryuhei Ozaki is a 5th grader who discovers a journal written by his father Shohei when he was himself a kid: "Tokyo Exploring Records,". Ryuhei informs his online friends and create his own exploration team. During the summer vacation, the three boys decide to go down a manhole into the sewers in quest of a hidden treasure indicated on the journal. Little did they know that the 'abandoned' sewers are hiding a very real treasure of the Japanese army, but also it's fierce guardian and a whole lot of weird 'undergrounders'.