Okay, I knew I wrote a summary awhile ago in the "Favorite Manga" thread on this series so I copied it and I'm pasting it here as well. Please note this only covers the first volume and the basics of the story. Others can add on if they like.
Daisuke Niwa is part of a family that has a "condition" that causes him to transform into Dark, the legendary Phantom Thief (his grandfather was once Dark as well). This condition happens whenever Daisuke longs for the one he loves which means it can happen at anytime and anywhere if he isn't careful. The only way to stop this from happening is if the one he loves returns the same feeling of love. Enter the Harada twins, Riku and Risa, and a large love triangle. (As of book 1) Daisuke can change into Dark, Daisuke loves Risa, Risa loves Dark, Dark loves Riku, Riku may be curious about Daisuke, and Riku thinks Dark is a pervert. Meanwhile, Satoshi, a young detective in Daisuke's class, is obsessed with one day catching Dark.
It's a good series. It has humor, romance, a little bit of action, and a little bit of drama. I'd recommend it.