In 2099, Eagon Must keeps the peace by erasing what makes people dangerous-emotion. His project, Helix, is almost ready to fold every mind into one obedient network. Then Susie-9 wakes in the ruins with a glitch no purge could burn out: hope. Hunted by Must's Black Echo enforcers, Susie falls in with Dorsey Axiom, the engineer who built the monster; Rafe, a soldier whose conscience chip cracked; Jax-11, a battered war machine; and the vanished genius Joseph Peter. Their target is the Sky Spine, the tower that runs the world. Their plan is impossible. If Susie can inject her "hope circuit" into Helix, it won't just break the system-it will teach it to feel. And feeling is the one thing Must can't control.