The Citadel unfolds on the protected planet of Dianis, where Achelous-the architect and orchestrator of the planet's defense against extrasolars-has been abducted by the powerful religious order known as the Paleowrights. Chained, tortured, and taken to the Empire of Nak Drakas, Achelous's fate remains uncertain.

Marisa, Achelous's mistress and a trader princess, embarks on a daring mission of restitution. To rescue Achelous, she must infiltrate the heart of her enemy-the Drakan Empire-and save him from Viscount Helprig. The Paleowright clergyman seeks Achelous's execution, not for what he knows, but for blasphemy against the Diunesis Antiquaria, the Paleowright's faith.

However, the commandant of the Drakan secret service is not so quick to condemn. He suspects Achelous is an Avarian, an agent of a galactic federation, the very people the Paleowrights revere as gods.

As corsairs raid the planet and Paleowrights conspire, Marisa races against time. Her success could trigger a global war, but can one man's knowledge be worth such consequences? The answer lies in what he knows.