Daisuke Niwa is a typical, albeit somewhat unfortunate, middle schooler. On his fourteenth birthday, he develops a "condition" that has afflicted the men in his family for three hundred years. Upon seeing his crush, Risa, he transforms into his alter ego, the phantom thief Dark Mousy. Unfortunately, when Dark Mousy sees Risa's twin, Riku, he reverts to Daisuke. The only cure for this madness is if Daisuke can get Risa to fall in love with him...