Takuya Aihara, the hero/heroine of the story. He's been blessed with a very interesting life: several years ago he was changed a girl through bizarre chemistry accidents, not once but twice. Now as he approaches his final year of university with faithful girlfriend Asuka at his side, he becomes nostalgic thinking about those crazy days he spent as a girl.
Until it happens again, that is: that troublesome Kouji, still madly in love with his female persona, secretly gets Asami-senpai to whip up some of the sex-change potion and tricks him into drinking it! It'll be harder than ever for him to change back into his male self this time, but he's got no choice; he has to find a way to return to normal before his female body becomes permanent. Although Asuka is one of the most understanding women in the world, there's no way she'll stay with him as a girl -- that's just not her thing.
As he starts another round of bizarre sexual adventures in his female body, virtually all the characters from the past are back: His twisted sister Natsumi, who loves to tease him; the ingenious Asami-senpai; the endlessly experimenting Chisato; and a host of others from the first two series.