Jack Oshinumi, a young Englishman, moves to Hokkaido seeking recovery from a severe lung illness. Resenting the relocation, he longs to return to Tokyo and his academic life. Soon after arriving, he experiences supernatural phenomena, seeing mysterious beings in dreams and waking life. An earthquake during a woodland walk leads him to Rin Funo, a family friend of his cousins. Rin gives Jack a protective horn bugle pendant. This artifact draws dangerous attention from factions in the fairy realm, Nymphidia. After the pendant is stolen, Jack enters Nymphidia through a portal in his room, the Shadow Moon Window.

Within Nymphidia, Jack befriends Pook, a shapeshifting stag-human creature who guides him. He discovers he is the human reincarnation of Gwyn, the former fairy prince. During the coronation of the new fairy king, the dark elf Queen Mab interrupts, attempting a coup. Jack uses the pendant, now transformed into a functional bugle; sounding it disperses Mab and her forces. The fairies proclaim Jack as Yōsei-Ō, the Fairy King. His coronation sets him on a quest to retrieve a magic ring stolen by Mab, vital to defeating her and restoring order.

Jack's journey takes him through forests, caves, and swamps, facing mythical adversaries like harpies, undines, and a unicorn-cyclops hybrid, overcoming them through combat and cunning. He reunites with Rin, revealed to be the Irish hero Cúchulainn, and their bond deepens. Securing the magic ring leads to a final confrontation where Jack defeats Queen Mab. This victory cures his illness. Returning to the human world, he finds significant time has passed. His experiences transform him from a solitary scholar into a resilient leader, forged by alliances across the fairy realm.