In a galaxy, darker than the pitch blackness of the universe, countless Netherworlds exist. In each Netherworld, there is an Overlord who reigns over its hot-blooded demons, and maintains the balance of the Netherworlds by keeping each other in check. But the sudden appearance of Demon Emeror Void Dark and his army, the Lost, has ravaged many Netherworlds, forcing them to surrender.

At the same time, there's been a young demon that stands all alone against the growing powers of the Lost. His name is Killia. He is an unknown demon that swore to take revenge on Demon Emperor Void Dark, and defeat the Lost that's been destroying various Netherworlds. Unexpectedly, he happened to save Princess Overlord Seraphina. Killia, accepted by Seraphina as a capable servant, has begrudgingly agreed to accompany her to overthrow Void Dark...