Long ago, a boy with a tail was rescued from a village and raised by humans. Years later, he now lives with a Succubus named Shiro and discovers his true identity as a Dominator - a monster with the ability to capture other monsters.
When enemy monsters are weak, he can use his power to shrink them and engulf them with his tail. Within his body he possesses a Mystic Sanctum which the captured monsters reside in a realm like a pocket dimension until he calls upon them. In exchange for their service, he offers them his bodily fluids - which as a Dominator are more nutritious and desirable than that of other men.
After becoming an adventurer, Shiro is captured by a mysterious masked man and Kuro sets on his quest to save her.