Daisuke Niwa is a clumsy, block-headed, and wimpy middle school student who has few redeeming qualities. On his 14th birthday, he finally decides to confess his love to his longtime crush Risa Harada, but is rejected. In an unexpected turn of events, however, Daisuke finds himself transforming into Dark Mousy, the infamous phantom thief, whenever his mind is set on Risa. Though Daisuke is unaware of this strange heritage, his mother is certainly not: since before the boy was born, his mother had been planning for him to steal valuable works of art and let the name of the elusive art thief be known.