The plot is a loosely based on a melange of motifs from previous Conan short stories, most notably "The Scarlet Citadel", with which its early chapters shares an almost identical storyline: Conan defeated in battle, captured and placed in a monster-infested dungeon, finds an unexpected ally and escapes; meanwhile, the population of the Aquilonian capital, believing him dead, riots and is ready to accept an alternative King. From here the two diverge: The Scarlet Citadel , a short story, ends with Conan coming back when the rioting just started and making short work of his foes; in the book-length Hour of the Dragon it is much more complicated, Aquilonia has to live under a long and harrowing foreign occupation while Conan goes through a long hazardous quest, before he could finally come back and dispose of his foes.
The book place when Conan is about forty-five, during his reign as King of Aquilonia, and follows a plot by a group of conspirators to depose him in favor of Valerius, heir to Conan's predecessor Numedides, whom he had slain to gain the throne. To accomplish this they resort to necromancy, resurrecting Xaltotun, an ancient sorcerer from the pre-Hyborian empire of Acheron. With his aid the Aquilonian army is defeated by that of the rival kingdom of Nemedia and occupied. Conan, captured, is slated for execution until the sympathetic slave girl Zenobia risks her life to free him.
Conan's quest to retrieve the Heart of Ahriman in order to defeat the wizard and regain his throne takes him through all the lands of Hyboria.
After his eventual triumph he vows to make Zenobia his queen.