Sunlight streams into the quiet bedroom. Waking up with a hangover, you feel a strange sensation on your body. Thinking it might be your wife's hand, you follow it and pull back the blanket—only to discover a shocking sight. Yui, your wife's younger sister who's been staying at your house for her job hunt, is lying naked beside you. Flustered and stunned, you barely manage to handle the situation without your wife noticing. But from that moment on, Yui begins sneaking advances whenever your wife isn't looking, one after another. Thus begins your slow surrender to her irresistible charm.