"My parents aren't home today." Minagi Ako, the protagonist's girlfriend, invited the protagonist over to be her "bodyguard" while her parents were away. Her parents would be away on a business trip for a few days... This was a clear OK sign for tonight, so the protagonist headed over to her house in high spirits. She suggested for him to take a bath, and while the protagonist was in the bath, the bathroom door suddenly opened. In the depths of the steam, his beloved was standing completely naked. Both were in the mood, and all the conditions were met. Their bodies joined together. The happy consummation of a young couple...or so it seemed. But then, the girl who had just lost her virginity, and only a moment ago had gazed upon him with moist eyes, suddenly changed. The corners of her mouth rose, and she murmured devilishly, "Serves you right, Ako (lol)." The protagonist was shocked to find out that the girl was actually Ako's twin sister, Minagi Mio. Having seduced the protagonist from her older sister, Mio basked in the feeling of superiority. Conversely, the protagonist was dismayed at having mistaken the person he loves. While living under the same roof, the younger sister secretly tries every trick in the book to seduce the protagonist. The older sister is puzzled by some things, but is excited about the idea of semi-cohabitation with her boyfriend. The days in which the protagonist tries to skillfully cajole the younger sister and to elevate his relationship with the older sister are about to begin. [Translated from the official website]