When the map begins reacting to signals no one else can hear, Tessa and Ollie are pulled beneath the city into forgotten tunnels, coded vaults, and a secret route known as the Founder's Underline. With every clue they unlock, an unseen force pushes closer - one that can bend memory, scramble trust, and turn their own tools against them. To protect the map and everyone they love, they'll have to risk the one thing they can't easily get back: each other.
