Ataru, a son, can't hide his excitement every day at his father's new wife. His stepmother is a young and cute girl named Chisa with H-cup breasts. Her body is so erotic that it's hard to know where to look. His father is busy with work and there are many days when he can't come home, so Chisa, frustrated, always relieves herself in the bath. However, Ataru, who knows this, can't endure just peeking, so one night he sneaks up to Chisa while she is sleeping...