A cart loaded with gunpowder explodes outside the General Letter Office, suggesting an attack on the institution responsible for carrying—and secretly intercepting—the nation's correspondence. Thomas Chaloner wants to investigate, but his employer assigns him instead to the poisoning of birds in the royal aviary.
The apparently trivial case leads back to the postal service, its surveillance operations and officials with access to private letters. When human victims begin to die from poison, Chaloner defies instructions to uncover an enemy using government intelligence systems against the state itself.