Oni Chichi 4 – When Kouzou Akizuki, the stepfather of two adolescent girls, announces a family holiday, his daughter Airi is quick to express her disdain for the idea. Eventually, though, she is brought around by her sister, Marina, who is looking forward to this opportunity to spend some romantic moments with their father. However, Marina will only join them a couple of days later, leaving Airi and Kouzou to their own devices.
Airi is aware of her father’s sexual inclinations and tries to avoid being alone with him in the same room. Unfortunately, she falls victim to a potent aphrodisiac during the train journey to the hot springs that Kouzou disguises as candy. Airi finds herself unable to control her sexual urges—which is exactly what her father has been waiting for.