One day, dungeons appeared all over the world. The creatures that emerged from the dungeons ravaged nearby humans and animals. However, it seemed that the UMA, commonly known as monsters, lost their power when they came out of the dungeons and were quickly exterminated by modern technology. Even in countries with insufficient weapons, chaos ensued, but the monsters that came out on the ground were weak and were eliminated by people who had been trained in their daily lives. Swift measures were taken, and the world regained its safety. Later, when a certain country discovered resources inside the dungeons, the world began to change. This is Japan. Finally, the dungeons were made public, and a series of humorous and touching episodes began. Behind the scenes, in a dilapidated house in the countryside, a dungeon suddenly appeared in the basement. The siblings, who had become impoverished and lived in the countryside after their parents disappeared, began to explore it for entertainment. This is their story.