Once again, the peaceful morning is shattered by a diatribe delivered by a sister to her brother. This is a daily occurrence at the Utsumi household, and the fact that his relationship with his step-sister, Miyu, isn't improving is a cause of anguish for Mitsuru. While she may behave like an angel in the presence of their parents, when they are alone, she's a pure devil.
One day, Mitsuru discovers that Miyu has a complex regarding her bust-size. He unwisely brings it up and she blows up at him. While attempting to restrain the raging Miyu, the two lose their balance and fall. Staring into Miyu's face up close, Mitsuru finds himself drawn by her cute features, the warmth of her body, the peculiar scent of a young girl...

(Source: Cranston, translated)