The protagonist comes to Tokyo for college entrance exams and reunites with his cousin Yui, whom he hasn't seen in a long time. She has grown into a mature, alluring woman, and he finds himself increasingly attracted to her. When their parents leave town for a trip, the two are left alone together at home. Unable to restrain themselves, they spend every available moment entwined in each other's arms. The story captures the gradual deepening of their once-innocent bond into something far more intimate.