Akira's father, an archaeologist, has remarried, giving him a new stepmother and three stepsisters: Yuki, Nana, and Miu. During his honeymoon, Akira's father makes an archaeological discovery and decides to remain on-site to supervise the work, leaving Akira at home alone with his new family members. After being caught in bed with Yukie, his stepmother, Akira finds a box containing a scroll and three bottles. Using the knowledge his father taught him, he reads the Sumerian text on the scroll and discovers he is bound by a contract with the goddess Inana. She instructs him to collect the love juices of his sisters in the bottles before the next full moon, warning that a terrible horror will befall him if he fails.