Daisuke Niwa is an ordinary middle-schooler until his fourteenth birthday, when a family trait kicks in: whenever his feelings for his crush Risa Harada surge, he transforms into the legendary phantom thief Dark Mousy. His mother Emiko puts the gift to use, sending Dark out to steal artworks tied to the Niwa line, while Daisuke must dodge the young police commander Satoshi Hiwatari, who carries a secret of his own.