"I know this scene all too well…!" Serizawa Eriya, a young man living a life of hardship, is left to support his remaining family member, his little sister, after his father died and mother fell ill. His life, which was already miserable from being scorned and treated almost like a slave by a politician's good-for-nothing son, takes a dramatic turn the day he clears a game called Valhalla's Cradle, his only source of solace. "The game I was so immersed in is becoming a reality?!" From the depths of the ocean, the final dungeon, "The Castle", emerges, and at the same time, monsters begin to ravage every corner of the land. The only way to stop them is to conquer The Castle, and Eriya is the only one in the world who knows how to do it. A story of turning the tables from the very bottom begins to unfold―!