The Citadel unfolds on the protected planet of Dianis, an Iron Age world, where Achelous, the architect of the planet's defense against extrasolars, has been abducted by the Paleowrights. Chained and tortured by the powerful religious order, he is carried off to the Empire of Nak Drakas where his fate is unknown.

After the mayhem and outrage of Achelous's kidnapping, Marisa, his lover and merchant princess, embarks on a mission of restitution. To rescue Achelous, she must go to the heart of her enemy, the Drakan Empire, and save him from Viscount Helprig. The Paleowright clergyman does not care what Achelous may know, just that he has blasphemed the Diunesis faith and shall be hung before the archbishop.

However, the commandant of the Drakan secret service is not so quick to execute. 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, Paleowrights conspire, and intervention of the Avarian Federation looms, 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.