Vampire Yui is the daughter of a male Shinma and a woman who possesses both Shi clan blood and vampire blood, specifically Vampire Miyu. Yui's specific Shi lineage creates a deep connection to a magical lake located on a different plane of existence. The lake's guardian, Yuki, is the sister of one of Yui's distant ancestresses. Over a century ago, Yuki fell in love with a mortal man and chose to enter a wakeless sleep with him instead of fulfilling her duty to control the lake's magic. The remaining Shi covet the lake and desire to use its power. They fear that Yui, who is naturally drawn to it, would awaken Yuki, who would then resume her rule.