Tsukikawa Keisuke had a strange dream. That dream involved his innocent younger sister, Saki. Saki was a naïve girl who even believed that babies were made from just kissing. However in his dream, Saki’s innocence was replaced by eyes filled with lust as she creeps into Keisuke’s bed. The next day, Saki was her same innocent self to Keisuke’s relief. He was relieved that it was a merely a dream.

However, he soon learns from his father that a succubus lurks within his little sister. Having to choose between his dear, innocent little sister and the allure of a lustful succubus, what will Keisuke do?