The story starts off with a 14 year old boy named Daisuke Niwa. Today is his birthday and he finally has enough courage to tell the girl he likes about his feelings. Risa Harada, his crush, though doesn't harbor the same feelings towards him. She promptly tells him that she can't accept his feelings and wishes they could still be friends.
He returns home depressed and heartbroken because of what happened. To make matters worse, he transforms into another person's body. The transformation was triggered by thinking about Risa. Only after the transformation does his family tell them about the Niwa family legacy. This legacy is turning into the legendary phantom thief, Dark-Mousy on their fourteenth birthday. The only way to change back is to steal the St. Maiden's Tears statue, or so they claim. No only that, he finds out that his pet, With, isn't all what he seems. He's actually Dark's partner and serves as his wings.
When trying to get the statue, he meets up with Satoshi Hiwatari, a schoolmate who happens to be the commander in charge of getting Dark. Satoshi pins him to the ground with causes Daisuke to freak out from the prospect of getting caught. This causes a chain reaction which awaken Dark who takes control. They manage to steal the statue and get out unscathed.
Of course this isn't the end for Daisuke. His life only gets more complicated from then on...