On Hōrai Island, a group of half-demon children watches in awe as a Kikyo look-alike steps out of a green orb above the Cauldron of Resonance. Four scars—the mark of the Four War Gods [Ryūra], [Jūra], [Kyōra], and [Gōra]—appear on the backs of all the children except Ai, the youngest.
The oldest, Asagi, urges Ai to leave the island, and Ai is rescued by Inuyasha, Kagome Higurashi, Miroku, Sango, Shippo, and Kirara. After Ai asks Inuyasha to save the others, he explains the island’s myth: Hōrai Island appears only once every fifty years. He recalls fifty years ago when he and Kikyo arrived, only to be ambushed by the Four War Gods: [Gōra] extracted Kikyō’s blood to fashion her doppelgänger, and [Ryūra] marked Inuyasha’s back with the four scars.
Following the island’s reappearance, Kikyo learns of the threat and heads to investigate, while Sesshomaru breaks from his group—Jaken and Rin—to face Kyōra. As Inuyasha and his companions near the island, Jūra arrives and fires his Thunder Cannon. Miroku and Sango fly on Kirara to deflect the attack, while Inuyasha, Kagome, and Ai escape to the shore and meet Ryūra. Kagome intervenes and saves Inuyasha from Ryūra.
Ai leads the group to a village where they discover the children and learn that Asagi will sacrifice herself to the Cauldron of Resonance, arguing that her death will buy a little more time for the others. They all follow Asagi to the Cauldron, and both Inuyasha and Asagi are pulled inside when he tries to destroy it. On the outside, the others and the children struggle to open the doors while Lady Kanade gives crucial history: she once tried to destroy the War Gods by taking their power spheres into the Cauldron at the cost of her own life, only to learn the gods now force half-demons into the Cauldron to slowly regain their power. She entrusts Inuyasha with a box containing the four power spheres and her remaining strength in exchange for him destroying the Cauldron and saving the children.
Inside, Inuyasha and Asagi break free and shatter the Cauldron, which also tears apart Kagome’s bow and arrow set. Then the Kikyō look-alike emerges, stealing the power spheres and releasing them to the Four War Gods. The group splits up: Inuyasha confronts the doppelgänger of Kikyo, Miroku and Sango ride Kirara to take on [Jūra] and [Gōra], while Kagome, Shippo, and the children attempt to fashion a raft to escape, only to be blocked by Ryūra. The real Kikyo arrives and destroys her doppelgänger, leaving behind her bow and arrow.
Back on the shore, Inuyasha battles Ryūra. Miroku, Sango, and Kirara defeat [Jūra], clearing the way for Inuyasha to confront [Gōra]. The power spheres inside the island converge to form a new War God, and the only way to defeat it is a coordinated strike: Kagome’s sacred arrow paired with Inuyasha’s Backlash Wave. The story reaches a resolution when Shion is guided by fireflies carrying sacrificed souls to find Kikyō’s arrow, and Kagome and Inuyasha unleash the combined power to destroy the final War God and lift the marks from those who bore them.
With the danger ended, the survivors flee the island on a raft as it sinks. The children stay briefly in Kaede’s village before departing to pursue their freedom, leaving behind a resolved myth and a sense of closure for Inuyasha and his friends.
