Train Heartnet, also known as "Black Cat," was an infamous assassin for a secret organization called Chronos...until he abandoned that cold-blooded existence to live on his own terms as an easygoing bounty hunter. But is Train's past as far behind him as he thinks?

In order to revert back to his regular-sized self, Train sets out to see Dr. Tearju, a woman who bears a striking resemblance to the girl Eve. But before she can help Train return to normal, the Apostles of the Stars come crashing in, with the intent of forcing Dr. Tearju to join their ranks. A mysterious new weapon may be the only hope for keeping the Apostles at bay.