A terrifying underground exploration

Hiroshi and the five of them form the detective team "Hadogumi" to take on Matenro's challenge. However, as they are elementary school students, they are busy with homework and other things, so it seems difficult to concentrate on their activities as Hadogumi. Just as they are already facing these worries, Ochiai-sensei comes and invites the five to play hide-and-seek. The five people hid under a manhole, but...