Shun Mikura was enjoying a carefree life with his father. One day, he was worried about the large amount of packages that had arrived at his house. his father suddenly announced that his mother and sisters would be moving in. His parents who had separated from each other, got back together. The protagonist is surprised to see his sisters again for the first time in five years, but tries to act nonchalant. However, on the night of their reunion, he witnesses his parents having sex. When he wakes up the next morning, he finds his newly reunited sisters in the futon, playing with his crotch, The protagonist is startled by the sight, but still ejaculates. The sisters observe the freshly ejaculated semen and confirm that the sperm is active. The protagonist wonders why the sisters are so surprised. "We both fell in love with Shun-chan." “So we decided together, we'll have your baby!" The one who can have Shun's baby first will become his wife!