Akira's father, an archaeologist, has remarried, giving him a new stepmother and three stepsisters, Yuki, Nana and Miu. While on his honeymoon, Akira's father makes an archaeological find and decides to stay on site in order to supervise and leaving Akira at home alone with his new family members. After being discovered in bed with Yukie, his stepmother, Akira finds a box containing a scroll and three bottles. Using the knowledge his father taught to him, he reads the Sumarian language on the scroll and finds himself in a contract with the goddess Inana, who tells him to collect the love juices of his sisters in the bottles before the next full moon or else a terrible horror will befall him.

(Source: Anime News Network)