The Iron Sword follows Robin Goodfellow, also known as Puck, as he continues his quest to save the Nevernever from a primordial threat. Alongside allies like Meghan Chase and Ash, Puck must confront the rising power of the Nightmare King and the shadows threatening to consume the faery realms. The story explores the origins of an ancient evil and the sacrifices required to maintain the balance between the Iron and Fey worlds.