Nineteen-year-old Wilhelm Dortmund is ready to shed his false identity as prince to the Agrestis throne. But after The Rooster's captain is mistakenly abducted in his place, Wilhelm instead finds himself with yet another role to play—that of the Iron Hedgehog.

Wilhelm dreams of the day he can leave the iron suit behind to find Agrestis's true prince. But when word reaches the catafal that the Celestial Servants are experimenting on the imprisoned captain, the marquess has no choice but to take the ship north to the desolate Heart of Ice. The Servants want royalty, and Wilhelm is willing to offer himself as trade—even if it means postponing his search, as well as his deepening romance with Fitcher Nightingale. But when duty and heart clash while a battle for Life and Death looms, the false heir of Agrestis will have to decide once and for all who he truly wants to be.