Having escaped Thorbury, Tamzin Pook and her patched-together crew reach Museion, a traction city trapped in a mountain valley and besieged by predator suburbs on every side. External sieges are the easy part: when people aboard Museion start dying one by one, Tamzin has to find a traitor before the city itself, hunted from without and rotting from within, comes apart on a storm-lashed bridge.
Summary by Loreful, from public sources