Is this another world? No, it's modern-day Japan!

Four years have passed since dungeons suddenly appeared all over the world, sparking conflict over the resources hidden within. Then one fateful day, a financial securities salaryman named Mizuki, who was relegated to an office branch in the countryside, arrives at his new workplace to find it transformed into one of those very dungeons. Entering to search for anyone who might be stuck inside, Mizuki realizes the dungeon is straight out of a video game!

During his search, he comes across a fairy and, using his new mysterious skill book, saves her from being roasted alive by goblins.

The former wage slave's adventure begins now, as he sets out to clear modern dungeons with his skill book and his partner from another world!