In recent years, an increasing number of girls have been experiencing the development of "tertiary sexual characteristics" during the later stages of puberty—specifically, the rapid masculinization of a certain part of their external genitalia. Ooi Haruka is one such girl, who has until now kept the changes in her body a secret from everyone except her family. However, after an embarrassing incident involving the guy she's been crushing on in a crowded train, she wakes up to find herself at school, surrounded by a group of students calling themselves the "Bra Girls (Swing Girls)." And they know her secret!?