In the third year of high school, Izumi Tohsaka reunited with his younger brother, Kaoru Yano. Seven years after my parents divorced, I decided to live together in a dormitory shared room again. Lower voice, red-dyed hair, mature look, unknown surname… Izumi is attracted to him who has changed everything and doesn't even realize he is his younger brother. Even though they are supposed to be siblings, this emotion in my heart…?