As an average middle schooler, Daisuke Niwa's luck takes a turn on his fourteenth birthday. Afflicted by a centuries-old 'condition' inherited by the men in his lineage, he morphs into the notorious phantom thief Dark Mousy whenever he encounters his crush, Risa. The twist? At the sight of Riku, Risa's twin, Daisuke reverts to his normal self. The quirky remedy to this dilemma lies in winning Risa's heart and breaking free from this whimsical cycle.