A murder occurs at "Yamatsukikan," a long-established Japanese inn. The young owner, Ruka Tsukiyama (Uehara Ruka), pleads with renowned detective Kou (Shuri Himesaki) to solve the case. That night, mysterious men attack Ruka when she refuses Kou's assistance. Gradually revealed are the Moon Mountain family's traditional rules, a stone shaped like the moon, and the existence of Ruka's twin sister, Megumi (Uehara Ruka). Amidst these revelations, another murder erupts! A chilling and captivating story unfolds, woven with intense developments and deepening mysteries.