Hourai is a mysterious island that appears for a limited period once every 50 years. Some say that the isle holds the secret of eternal youth; others believe that it brings disaster to their land. When Inuyasha's group saves a half-demon child from being hunted by a monstrous creature, they learn that her name is Ai and that she fled from Hourai. Surprisingly, the girl is an acquaintance of Inuyasha and begs him to rescue her friends, who are being held prisoner by vicious demons calling themselves the Four War Gods—an enemy Inuyasha remembers well.

Upon reaching Hourai, Ai's guests are horrified to discover that only a handful of orphans still inhabit the island and are destined to become sacrifices to increase the Four War Gods' powers. Furthermore, the victims bear their oppressors' enchanted mark, which prevents them from escaping. Defeating the four demons remains Inuyasha's most viable option, but their superiority in strength will require daring initiatives achievable only by the group's combined teamwork.