Numerous dungeons suddenly appeared in the world. In this era, military personnel and civilians known as “Explorers” venture into these dungeons to search for mysterious treasures and bring them back. Yuma Mitaka, a second-year high school student who lived a life completely unrelated to dungeons, one day discovers a hole that mysteriously appeared in a corner of his backyard. Peering inside, he finds a creature with a metallic body trembling. At first, he couldn’t understand what it was, but soon realizes that it’s an extremely rare monster known as a Metal Slime, seldom seen in the world. Confused by the situation, Yuma wonders, “Could this be a dungeon?” Despite his bewilderment, he starts a peculiar new life, diving into the world’s smallest dungeon and continuously defeating Metal Slimes.