A homeless orphan is found under a bridge by Lord Takamaru Rentaro Mikuni, the patriarch of the wealthy Yamabe family, and is named Onimaru Yûsaku Matsuda. Takamaru brings the boy to Higashi-no-sho, a desolate estate near the volcanic Fire Mountain, where a long tradition of Shinto ritual is led by a priest who worships a serpent god. The estate itself is split into two feuding clans, with the east mansion controlled by Takamaru and the west by Nishinosho, each side vying for influence and prestige within the family’s tightly wound world.
Onimaru grows up alongside Takamaru’s two children, Hidemaru Nagare Hagiwara and Kinu Yuko Tanaka. Hidemaru, entitled and elitist, treats the adopted boy with contempt, resenting the favored status he receives from their father. Kinu, by contrast, forms a bond with Onimaru that blossoms into a complicated affection as the years pass. Onimaru becomes a brooding but charismatic figure, and his intense presence casts a shadow over the family’s already fraught dynamics.
As the children come of age, the family’s world grows more rigid and dangerous. Takamaru reveals a brutal fork in their lineage’s fate: the women of their clan who reach maturity must serve as priestesses at the mountain shrine. Kinu rejects this fate, craving freedom, and in a bid to escape, she marries Mitsuhiko Tatsuo Nadaka, a man from the west mansion who fits her social standing. The night before her wedding, she nonetheless gives in to a moment of temptation with Onimaru, a betrayal that fuels his rising fury and longing. This reckless passion becomes a defining wound in the house’s history, and Onimaru sets out to secure Higashi-no-sho for himself and to win back Kinu.
Hidemaru leaves for the city, where he carves out a life, marries Shino Keiko Itō, and starts a family whose son is Yoshimaru Masato Furuoya. The peace of the estate is shattered when Takamaru is murdered right before Onimaru’s eyes by a band of trespassers, an event that triggers a violent scramble for power and inheritance. In the wake of Takamaru’s death, Hidemaru returns with a new sense of purpose, determined to reclaim his birthright, and his arrival disrupts the fragile balance of Higashi-no-sho as Onimaru’s hold on the estate is challenged.
A few years later, Kinu bears a child while Onimaru has risen in skill and ambition, gaining the favor of the Shogun and a local lordship. Yet for Kinu, love has grown complicated: she remains torn between Mitsuhiko’s gentleness and Onimaru’s consuming obsession. Tragedy deepens when Shino is murdered after a brutal assault by bandits, and Hidemaru melts into a broken alcoholic, eventually wandering into a forbidden village where he is killed. With these losses, Onimaru tightens his grip on Higashi-no-sho, treating Yoshimaru as little more than an indentured pawn under his control.
Tae Eri Ishida, Mitsuhiko’s sister, comes to work at the east mansion and confronts Onimaru with a ruthless honesty, suggesting that his jealousy proves he is “more man than demon.” Her words light a fuse, and Onimaru’s violent response culminates in Tae’s suicide. Simultaneously, the Nishinosho clan is attacked by the same force of bandits that were hired by Onimaru, revealing his complicity in their assault. Mitsuhiko is killed in the ensuing conflict, and Kinu gives birth to their daughter, then dies after a difficult labor. In her final act, she entrusts her daughter with her name, and Onimaru, haunted by the memory of Kinu, exhumed her body to hold on to the past.
Twelve years pass, and the once-demonized Onimaru rules Higashi-no-sho with a heavy, unhinged control. The young Kinu Tomoko Takabe, now grown into a capable if disillusioned young woman, works as an orphaned servant under the increasingly erratic estate lord. She falls for Yoshimaru, now grown and furious at his uncle’s oppression, and the pair secretly plot to overturn Onimaru’s autocratic rule. Yet Onimaru remains haunted by the mother who gave him life and the girl he could never truly possess.
The feud comes to a fever pitch when Onimaru drags the coffin of Kinu’s mother—the body of the elder Kinu—into Higashi-no-sho, intending to desecrate it as part of a final, reckless act. Enraged, the younger Kinu rises against him, and Yoshimaru draws his sword, severing Onimaru’s right hand. The pair expel the tyrant from the estate as Fire Mountain trembles and finally erupts. In the climactic blaze, Onimaru takes the coffin into the crater’s molten maw and vanishes, leaving the future of Higashi-no-sho uncertain as the mountain roars in the background.
