Miyagawa Atsushi is an 11-year-old boy. Since he is 180 centimeters tall and has a grown-up looking build, he has always been running into troubles. On the other hand, his 17-year-old elder sister Atsumi is only 137 centimeters tall and has a childish looking build. Atsumi takes good care of Atsushi, who acts his true age.