After years of pretending to be human, the changeling Edmund Carter has assumed his rightful role as Cinder, king of the World Below. But not everyone at the royal palace is happy about his return.

Meanwhile, Ed is adjusting to human life in the World Above. His birth family treats him with a kindness he never knew growing up in the Fay court, but Ed misses the sense of purpose he had as a knight.

When a mysterious new threat emerges in the World Below, Cinder must call on Ed and their older sister, Alexis, for help. But nothing can prepare them for the family secret that awaits at the end of their perilous quest.