Because of his father's new job, Yahiro-kun was forced to move with his sister Yo-chan to a new apartment. They quickly adapted to their new abode and began to get used to independent life. However, soon after enrolling at the academy, Yahiro noticed a strange large truck outside his house. Yo-chan's belongings had long since arrived and he wondered what was going on there. Climbing up the stairs and fearing that there might be a burglar in the unlocked room, Yahiro began to slowly open the door. There he met a half-naked girl, dressed only in her underwear, zealously pushing a heavy piano. On seeing our hero, the girl screamed and forced him out. A little later she put on a beautiful dress and came out to say hello. Her name was Ruria Fondina and she was a princess of one of the island states. The girl had arrived to live with our hero. Soon other "princesses" began to appear at Yahiro's door...