Cu Chulainn, a knight from the fairy realm of Nymphidia, bursts unexpectedly into the human world. Riding through a window on horseback during Midsummer's Night, he arrives to rescue a sickly boy named Jack. He instructs Jack to open a "moonlight shadow window," a portal to Nymphidia, revealing its inhabitants await Jack as their new Fairy King and spiritual successor to Gwyn ap Nudd.

An immediate, instinctual connection exists between Cu Chulainn and Jack. Upon their first meeting, Jack intuitively knows the knight's name and feels a sense of prior familiarity. Within Nymphidia, Cu Chulainn acts as Jack's guide and protector as the boy navigates his new role and the realm's challenges. He shares a close association with Queen Mab, ruler of the dark elves, having been both her lover and the best friend of the previous Fairy King, Gwyn. This dual connection creates tension around trust, forcing Jack to reconcile Cu Chulainn's past loyalties with his current support.

Cu Chulainn aids Jack in confronting threats to Nymphidia, battling harpies named Ocypete, Aello, and Celaeno, and assisting in the rescue of a maiden from the merrow Melusine. The deepest conflict arises when Jack must confront Queen Mab herself. Throughout these events, a shounen-ai undertone develops in Cu Chulainn's relationship with Jack, implying mutual romantic feelings not explicitly depicted. This contrasts with the unrequited feelings held for Cu Chulainn by Ihika, Mab's half-brother.

By the narrative's conclusion, Cu Chulainn fulfills his role in securing Jack's acceptance as Fairy King. His bond with Jack is further emphasized through Puck, a male Ezo deer spirit, who expresses a desire to remain permanently as Jack's companion.