Born into a new world after a life of stagnancy, Will awakens to the faces of a skeleton, a ghost, and a mummy. Living in the ruins of a city long fallen, the three raise Will as their own. The skeleton— Blood—teaches him to fight; the ghost—Gus—teaches him magic; and the mummy—Mary—teaches him religion and responsibility. Most importantly, they all teach him love. As Will grows up and learns about the world he was born into, he prepares for the day when he must finally set out on his own. For Will, this journey includes a lifelong promise. At their coming-of-age, every adult is required to swear an oath to the god of their choice, with the strength of the pledge affecting the degree of their sworn god's blessing. With his departure approaching, Will must prepare to accept the truth of his undead guardians and embark into a world that even they don't know the state of. Will discovers, however, that every oath must be fulfilled, one way or another.

Two years had passed since he left the City of the Dead, and Will was seventeen by count. As a lord, he developed "Torch Port, a river port of light", and gradually the people's activities and smiles returned to "Beast Woods". However, out-of-season flowers bloom profusely, and an abnormality is discovered in the forest. Will and his friends head into the depths of the forest to solve this problem, and receive an ominous prophecy from the king of the forest. "In the Iron Rust Mountains, the 'Fire of Black Calamity' will occur. The fire will spread, or it will burn everything in this land." What is the calamity that sleeps in the ruined dwarven city of Tetsusabi Sanmyaku...!?