In a world not too dissimilar from our own there exists a mysterious phenomenon where people can rob each other of aspects of their beings. If one wanted they could steal a few centimeters to become that much taller. If one wanted to acquire a new skill they could simply pick it off from someone with the desired proficiency. And if someone wanted to be an entirely different person then their new self is ripe for the taking and reality would be twisted into making it as if it had always been them all along. The catch to pull off such feats however is that in order to take from someone, they have to have been taken from to begin with.
How this phenomenon began and the true identity of the very first usurper is lost to history and is a subject of myth but aspect thieves remain a plague on the society of this world. The limitations vary from person to person and most are incapable of stealing anything more than a trivial trait but every single theft only leads to more victims. To counter this blight on their very selves, investigators and enforcers have risen to track down such wrongdoers and make the victims whole once more.
This story follows a capable young unknown and the famously prodigious and narcissistic highschool girl detective to whom he is assistant to. Together they pursue the mysterious "Moral Thief", a serial aspect thief who robs people of the justice in their hearts in order to turn them into criminals. They do not know what motives he may have for this but they chase after this elusive thiefmaker all the same in order to find uncover the truth behind the assistant's lack of identity.
After having been robbed of his very self, all the assistant had left to him was the thiefmaker's calling card and the torturously vast void in him that never ever goes away. He didn't even have a name to go by and no one would ever come to recognize him for who he was before but as fate would have it, a great detective happened to be there at the scene of his loss with him. She explains to him that although she herself can't remember who he was, she still remembers that a victim would come out of all this and that she'd do everything in her power to help him and atone for not having stopped this most unforgivable of thefts.
Using all their wits and talents at their disposal, the pair help out the biggest victims of this phenomenon and go on twisting adventures through their city as they come ever closer to the truth and realize that their relationship is more than mere detective and assistant but also that of thief and victim.