Many dungeons suddenly appeared all around the world. It was the era in which both civilians and the army were exploring the dungeon as ‶Seeker″, and brought back the miraculous items with them. On a certain day, Mitaka Yuuma [2nd Year High School Student] whose life had nothing to do with a dungeon found a hole in the corner of the garden of his house. When he tried to peek into the hole, he found a metal body trembling non-stop inside the hole. At first, he had no idea about the hole he found but knew that the trembling metal body he found was a monster called metal slime that had become the current hottest topic in the world. 「Could it be that this hole is actually, a dungeon?」 was the conclusion that he came with thus, began the change of his daily life, coming into the dungeon hole and beating the metal slime.