It has been several decades since dungeons made their appearance on Earth, bringing along rampant monsters. Due to the contamination of the mana caused by the dungeons, people gained powers called "skills", but it is not like the adventures that people see in tales and legends. Many towns were abandoned, the economy declined, and the world was filled with lawless areas, slowly inching towards extinction of the human race. In a ruined town where death is always around the corner, Amano Mamoru continued to hone his skills day and night. Together with the mysterious girl Shizuku, with whom he shares the same fate through the skill "Remaining Lives", and with his unique skill "Equivalent Exchange Store", he will challenge the world's coexisting dungeons!