In recent years, more and more girls are experiencing the development of "tertiary sexual characteristics" during the later years of their puberty -- that is, 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 but her family. However, after an embarrassing incident involving the guy she's been crushing on in a crowded train, she wakes up to discover herself at school, surrounded by a group of students calling themselves the "Bra Girls (Swing Girls)." And they know her secret!?

In recent years, more and more girls are experiencing the development of "tertiary sexual characteristics" during the later years of their puberty -- that is, 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 but her family. However, after an embarrassing incident involving the guy she's been crushing on in a crowded train, she wakes up to discover herself at school, surrounded by a group of students calling themselves the "Bra Girls (Swing Girls)." And they know her secret!?