The Ushiromiya family gathers on Rokkenjima, a small island off the coast of Japan, for their annual family conference to discuss how the inheritance of the ailing family head, Kinzo, will be distributed among his children and their spouses. Battler, who has not attended the conference in six years, notices a new painting in the foyer of the mansion. He is told that it is a portrait of the "Golden Witch" Beatrice, who had supposedly given Kinzo all his wealth, and who Kinzo desires to see again. Battler, however, does not believe in witches or magic, and doubts she exists. A typhoon approaches the island, and Maria is left outside by her mother Rosa while searching for a wilted rose George had marked for her in the garden. When it begins to rain, Battler, Jessica, George, and Rosa go outside to find Maria with an umbrella she did not have before, which Maria says Beatrice gave her.

Rosa's siblings and their spouses go missing, while the remaining guests find the chapel door locked. Finding the key inside the envelope Beatrice gave Maria, everyone enters and find the missing people murdered inside, along with some of Kinzo's hidden gold. Jessica and Kanon are ambushed by Beatrice, who summons her legion of goat servants against them. Kanon fights off the goats until Beatrice summons two Stakes of Purgatory, Asmodeus and Satan, who kill Jessica and Kanon. In Purgatory, Battler and Beatrice suspend their game to determine how the first six people could have been murdered inside a locked room. Battler begins forming several theories, but Beatrice counters most of them, using red text to speak the truth. However, Battler manages to catch Beatrice off guard by theorizing that the culprit could have taken the key from the envelope, locked the dead bodies in the room, and put the key back where it was. Beatrice congratulates Battler, but remains confident he will submit.

Beatrice and her predecessor engage in a magic battle, though a surprise attack allows Beatrice to kill her teacher. Battler is bewildered by the entire spectacle, but regains his composure when the former Beatrice, under the new name Virgilia, reappears and gives him advice for countering Beatrice's claims, allowing him to deny that the witches' battle ever took place. In the real world, the siblings discover the bodies of Kinzo and the servants, each locked in a room containing the key to the next locked room, forming one large locked room. With Virgilia's guidance, Battler theorizes that one of the six victims is the murderer, created five of the locked rooms, and then died in an accident in the sixth room, forcing Beatrice to concede this round. Meanwhile, Eva, urged on by the vision of her younger self, solves the riddle of Beatrice's epitaph and discovers Kinzo's hidden gold.

Kinzo appears at the family conference and denounces his children as being unfit to succeed him as the head of the Ushiromiya family. Deciding that his grandchildren may be more suitable successors, Kinzo summons the Siesta Sisters to select the first six sacrifices for his ceremony to revive Beatrice (Natsuhi, Eva, Hideyoshi, Rudolf, Rosa, and Genji), while the survivors (Krauss, Kyrie, Shannon, Kanon, and Nanjo) are imprisoned in the dungeon of Kuwadorian by the demon Gaap. Afterwards, Maria has her revenge against Rosa for destroying Sakutaro by torturing her with Beatrice's assistance, and fully realizes her own potential as a witch. Gohda and Kumasawa, who had survived the slaughter and escaped capture, inform the cousins in the guesthouse about what happened when they receive a phone call from the hostages telling them to stay in the guesthouse until Kinzo makes preparations to test his grandchildren for the family headship.