Middle school student Rinichi Sagara has always been troubled by his short stature and feminine features. Criticisms of his appearance easily provoke him into fights, and as a result, he has earned himself a reputation as a surly and scary individual. Most of his classmates are unwilling to approach him, except for the tirelessly kind Aya Kurokawa and Yuu Narumi, who is Sagara's rival for Kurokawa's affections. One day, while the two boys are once again arguing over the girl they like, Sagara suddenly collapses.

When Sagara regains consciousness, he discovers that he has transformed into a girl. In this world, to maintain an equal ratio of males to females, some boys randomly change sex through a phenomenon known as "emergence." Refusing to accept his new status, Sagara becomes even more determined to behave as masculine as possible. Despite his efforts, both Kurokawa and Narumi soon learn of his secret—and this knowledge dramatically upsets the previous dynamic of their love triangle.

Volume 2: S no Kaikoroku (Memoirs of a Sadist)