Two years ago Saitou Hikaru's much shorter sister had a crush on her brother and promised to be his bride when she grows up. Hikaru played along with this innocent crush. Now, years later, his middle school sister Fuuka, nicknamed Saipuu (cyclops), has become very much taller and busty, still revealing only one eye due to her unconventional hairstyle. With Saipuu's model-like figure, her seemingly innocent infatuation with Hikaru becomes quite embarrassing and leads to many misunderstandings at home and at school.