The Houses in turn are ruled by the Emperor, an impossibly powerful, immortal necromancer whom they have worshipped as a god for the past ten thousand years.

At the start of Gideon the Ninth, the Emperor invites the heirs of the Nine Houses and their sword-wielding bodyguards (called cavaliers[2]) to undergo a series of trials to become Lyctors.

Her plans of fleeing to join the Emperor's armies (called the Cohort) are quickly foiled by her lifelong antagonist and heiress of the Ninth House, Harrowhark "Harrow" Nonagesimus.

Despite their clashing personalities and mutual hatred of each other, Gideon is Harrow's only real choice of cavalier, primarily due to a purported atmospheric contamination incident around the time of their births that killed the rest of the Ninth House's children.

After one such prolonged absence, Gideon goes looking for her and, with the help of the Sixth House, finds her in a hidden basement containing a plethora of necromantic experiments left by the Emperor and his original group of Lyctors.

Harrow reveals that the atmospheric contamination incident was an intentional human sacrifice by her family in order to create a necromancer powerful enough to save the Ninth House from economic and political failure: herself.

(An infant Gideon somehow survived the sacrifice, leading Harrow's parents and the rest of the Ninth House to treat her coldly out of fear.)

"[7] Jason Sheehan's NPR review said of the novel's genre: "Gideon the Ninth is too funny to be horror, too gooey to be science fiction, has too many spaceships and autodoors to be fantasy, and has far more bloody dismemberings than your average parlor romance.

Harris said that Gideon is a "fiendishly talented fighter who is forced to use new and unfamiliar tools, and has to sacrifice some measure of subtlety in favor of crude effectiveness.

[21] It has received endorsements from authors V. E. Schwab, Charles Stross, Robin Sloan, Warren Ellis, Martha Wells, Amal El-Mohtar,[22] Kiersten White, Annalee Newitz, Genevieve Cogman, Kameron Hurley, Django Wexler, Yoon Ha Lee, Rebecca Roanhorse, Richard Kadrey, Rin Chupeco, Max Gladstone, and Brooke Bolander.

[23] Writing in The New York Times, El-Mohtar called Gideon the Ninth a "devastating debut that deserves every ounce of hype it’s received" and praised it as "deft, tense and atmospheric, compellingly immersive and wildly original.

"[24] Liz Bourke and Carolyn Cushman gave a more critical review in Locus, saying that the novel failed "in its interrogation of the central hate-co-dependence-need relationship between Gideon and Harrow.