On 25 November 1937, at a former honjin in Okayama, the wedding of Kenzou Ichiyanagi and Katsuko Kubo is held. The celebrants include the mother Itoko, the third son Saburo, the second daughter Suzuko, the cousin Ryousuke, and Ginzo Kubo, Katsuko's uncle. During the ceremony, Suzuko plays the koto, and everything ends without incident.
Later that night, the wild sound of the koto is heard across the mansion. Ginzo rushes to the newly wedded couple's bedroom, only to find the couple killed in a brutal fashion. A Japanese sword is later found thrust into the ground in the middle of the garden, with no footprints on the surrounding thick snow, creating a perfect locked room mystery.