Set in the near future, it tells the story of Ruby, an ordinary girl, who became a treasure hunter after her life is changed dramatically when her father, who was also a treasure hunter, dies. Her objective is to find O-Parts: magical items hidden in ruins which grant people superhuman powers and can only be used by an O.P.T. (O-Part Tactician), Angel, or Devil. She soon meets a mysterious boy named Jio Freed who, due to having a dark, lonely past, seeks to conquer the world. Jio is hostile to her at first, but ends up traveling with Ruby as her bodyguard. When Ruby is attacked by an O.P.T., who claims to be Satan, Jio rushes to her rescue and a battle occurs. Initially they are on the losing side, but Jio releases his true power and is revealed to be not only an O.P.T., but the real Satan. Thus, the two continue to travel together in hopes of unlocking their pasts
