Yet again, the peaceful morning is shattered by the diatribe delivered by way of a sister to her brother. This is the daily occurrence on the Utsumi household, and the fact that his relationship along with his step-sister, Miyu, isn't improving is a source of anguish for Mitsuru. While she may work like an angel in the use of their parents, when they are alone, she's a pure devil.

1 day, Mitsuru discovers that Miyu carries a complex regarding her bust-size. He unwisely brings it down and he or she expands 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 temperature of her body, the peculiar scent of a young girl...