Tenya and Yamato Hagiwara were raised as twin brothers, but their entire relationship is thrown into turmoil when they discover the truth that they are not actually related by blood. In the wake of this revelation, the younger brother Yamato begins skipping classes and falling in with the delinquent crowd at their school. The older brother Tenya takes it upon himself to correct Yamato’s behavior and steer him back onto the right path, but his efforts are repeatedly met with difficulty and resistance. One day, a friend of Yamato’s named Tatsumi tells Tenya where he can find his brother, but the information turns out to be a trap. Tenya walks in on Yamato in a compromising sexual situation with another person. After the interruption, Yamato makes a shocking demand: because the girl who was with him ran off, Tenya must take her place as his replacement. The story unfolds within a school setting, focusing on the twisted and intense relationship that develops between the two former brothers as they navigate this coercive and taboo dynamic.