「I'll sell you the way to defeat that monster for 300,000 yen!」

――This is a story from not long after dungeons first appeared in modern Japan.

The hero "Takuto Ichijou," who made a name for himself in another world, returned to modern Japan, but he had lost both the magic power and physical abilities he had cultivated there.

What remained were only things useless in modern Japan, such as monster-slaying experience, knowledge of magic, and otherworld language ability.

Takuto began living as an ordinary person, but the days he couldn't utilize any of his abilities were painful.

Just then, labyrinths and monsters appeared in modern Japan. They were exactly what Takuto had seen countless times in the other world.

Three years later, Takuto, who finally obtained a national license to operate in labyrinths, abandoned stability and peace to head to the labyrinth, where he believed he could utilize all of his abilities.

Triggered by his encounter with the otherworlder "Filia," Takuto realized that his otherworld experience was incredibly beneficial to other adventurers who were practically beginners.

Eventually, Takuto, together with Filia, began selling methods to defeat monsters, tips for labyrinth exploration, and how to use magic, sometimes directly and sometimes through video streaming, turning them into money.

Furthermore, he invented useful items for labyrinth exploration, as well as "Status Cards" that visualize adventurers' abilities.

Their activities became crucial in Japan during the dawn of dungeons, eventually evolving into a public institution――.