Vintro is one of the youngest members of the Venari, an organization dedicated to protecting humanity from within the City from monsterkind. His parents are top hunters in the Venari, and he himself is considered a prodigy due to the ease at which he took to wielding Angelus, a special sword that harms monsters but not humans. He has never known life outside of the Venari, so normal teenager things are foreign to him.

His life gets thrown for a loop, however, when a mission goes awry and he finds himself bleeding out in the street. However, he is immediately saved by the appearance of a monster girl his age, who introduces herself as Crimson and uses a mysterious power to link their lives together, saving Vintro's life and transforming both of them into something neither entirely human nor entirely monstrous...