Young couple Takashi and Kannon have been married for half a year, they always wanted to have a baby but Kannon couldn't because she suffered from infertility. Coincidentally, Megumi - a flight attendant and Kannon's older sister, also wants to have a baby while the two couples cannot do it together regularly due to work circumstances. The story begins when Megumi visits her younger sister's house, and upon learning that Takashi is saving up a large amount of sperm for his wife, she seduces her brother-in-law to help her because it was the day she was ovulating. Despite continuous refusal and advice that what she was doing was completely wrong, she didn't care, instead she aggressively touched his private parts and even forced him into intercourse when her sister wasn't paying attention. This will continue to happen on the ovulation days of the next month if this lascivious sister is not pregnant.