Beneath Istanbul's streets, Gabe follows stories of cisterns, hidden passages, layered empires, and objects that traveled below official routes. Deniz knows the underground spaces adults underestimate - and exactly how much trouble one lantern can cause when it lights a path no map admits exists. The triplets enter a city below the city, where stolen pages, private warnings, and corrected histories once moved through darkness. When an unfamiliar mark appears beside one tied to Sol, the family must decide whether the underground route is warning them away - or warning them that someone else is close.