Kotone's husband, Takashi, is currently living apart from her. During this time, Takashi's divorced older sister, Natsumi, temporarily moves into the family home of Kotone and Takashi. With both parents already deceased, Kotone and Natsumi begin living together alone. One night, Natsumi hears moaning sounds coming from Kotone's room and feels a sudden excitement. From the next day onward, Natsumi's intense gaze constantly fixed on Kotone begins to shift her behavior gradually. Kotone starts noticing these changes, and their relationship begins heading in a new, intimate direction.