Thanks to his father's sudden remarriage, Tojo Basara finds himself with two beautiful new step-sisters: Naruse Mio and Maria.

After learning that Mio's foster parents were murdered by powerful demons hunting Mio because of her lineage, Basara reveals himself as a member of the hero clan - an enemy of the demons - and vows to protect Mio as a member of his family.

Basara's childhood friend Nonaka Yuuki, a fellow member of the hero clan, warns him not to get involved with Naruse Mio. She claims that Mio will bring great disaster, but Basara proclaims that he will protect Mio as a girl on his own.

Basara suffers a deep wound in the battle against the white-masked man after Mio's power. Mio regrets getting Basara involved, and challenges the white-masked man to battle on her own.

With the battle against the white-masked man settled, they returned to their bawdy daily life. Basara wakes up with his hand groping Maria's butt, he runs into a naked Mio when he enters the bathroom, and Yuki sneaks into the bath with him.

Basara goes on a date with Yuki to the shopping center. When the two of them are overwhelmed by a clerk's sales talk, Mio stops following to rescue them.