Shuu Abe wakes up in a strange place—at the fiftieth underground floor of the Ootsuka Metro, a labyrinth of metro beasts. The city of Tokyo has been transformed by the "Metro Overflow" and the "Super Fungus Pollution" for the past 100 years.

"It's like you were thrown into the last floor of the dungeon at level 1..."

But even so, Shuu and his partner, the chipmunk (Carbuncle race) Tamiko, stand up for themselves. They eat the Sporangium of defeated metro beasts to level up and learn "fungal skills." From the weakest to the top, this is the beginning adventure of one person and one small animal.

"New Tokyo," the land of thread people, has been rebuilt after the catastrophic collapse of the "Tokyo Tribunal." This is the adventure of a hunter spun into a new world.