In the aftermath of their climactic duel, Battler Ushiromiya is left to ponder the abrupt resignation of the Golden Witch Beatrice, who is now a husk of her former self. Beatrice's drastic departure from her defiant motives urges him to question every circumstance in the previous four games. Despite wanting to stay by her side, Battler resolves to regain control of the current situation, where Lambdadelta and Frederica Bernkastel have temporarily taken their places to play the fifth game board.

With the passing of Kinzou Ushiromiya, Krauss Ushiromiya succeeds the family headship amid deep financial troubles. Due to poor investment choices, the family inheritance is set to fall into the hands of Krauss' benefactors, allowing his siblings to easily take legal action against him. In order to assist her husband, Natsuhi Ushiromiya enlists Beatrice's help to revive the former head—hoping to conceal the truth until the debts are paid.

Enter Erika Furudo: a self-proclaimed detective who washes up on the island of Rokkenjima at the start of the family's annual conference. Welcomed as an honored guest, Erika quickly takes an interest in the ongoing discussion about the riddle of the epitaph. The pompous girl's tenacious investigation into finding the hidden gold, however, sparks a chain of events that threatens to collapse the order of the Ushiromiya family.