In the secluded mountains of the Tohoku region lies a village named Nanatsuki. Drawn by tales of a peculiar local legend, two curious students from a Tokyo university decide to investigate. Their journey, however, takes an unexpected turn when a blizzard engulfs them. Fortunately, they're rescued by Benio, the second daughter of the influential Nanatsuki family, who welcomes them to their imposing black mansion, Shichitsuki-kan (The Mansion of Seven Possessions). As the students settle in, they discover that the Nanatsuki family is amid an unusual wedding ritual. The bride must spend a night inside a coffin, and the marriage is deemed official if dawn arrives without incident. But as the ceremony progresses, it's abruptly disrupted by an ominous force, casting shadows of doubt and suspicion on everyone present, including the ten attendees and the two outsiders. As dawn breaks on this isolated island with no escape route, the aftermath of a grim tragedy unfolds.