Akira Kohinata is the central protagonist. His archaeologist father remarries Yukie Kohinata, who becomes Akira's stepmother and brings three stepsisters into his life: Yuki, Miu, and Nana. The family moves into a new residence together.

When his father stays abroad to oversee an archaeological discovery instead of taking a honeymoon, Akira finds himself unsupervised in the household with his stepmother and stepsisters. During this time, an intimate encounter occurs between Akira and Yukie, leading to their discovery together in bed. This event prompts Akira to discover a mysterious box in his father's study.

Inside the box are an ancient scroll written in Sumerian and three bottles. Using knowledge passed down from his father, Akira deciphers the scroll. This act binds him into a contractual agreement with the goddess Inana. The goddess tasks him with collecting the "love juices" from each of his three stepsisters into the bottles before the next full moon arrives. Failure will result in severe consequences.

Akira's subsequent actions and decisions throughout the narrative are driven by this supernatural obligation and the urgent deadline imposed by the contract.