Gideon the Ninth (2019), the debut novel by Kiwi author Tamsyn Muir, is a work of sci-fi fantasy told through the perspective of the 18-year-old swordswoman Gideon Nav. It is the first book of the bestselling Locked Tomb Series. Gideon is an orphan enslaved by the Ninth House, the last of Nine Houses that serve a necromantic Emperor who is revered as God. Muir combines the magic of necromancy with traditional elements of sci-fi, such as interstellar travel and spaceships, and uses Gothic aesthetics along with the genre conventions of space opera and grimdark to explore themes of guilt, sin, redemption, and LGBTQ+ relationships. Gideon the Ninth won many awards, including best book of 2019 from NPR, the New York Public Library, Amazon, and more. This guide uses the 2019 Tom Doherty eBook edition of the novel.

Other work by this author includes the novel, Harrow the Ninth.

Content Warning: Gideon the Ninth contains graphic violence and gore, death, self-harm and suicide, and physical and emotional abuse.

Plot Summary

Gideon the Ninth takes place in an unspecified star system with nine planets revolving around the star Dominicus. Necromancy (death magic) is common in this world. Some individuals are born with the ability to manipulate thanergy (death energy) and thalergy (life energy); they can convert thalergy into thanergy but not the other way around. Necromancy is conducted through “theorems,” (similar to mathematical formulas) that achieve certain effects, and there are devoted schools of study for necromancy’s individual branches. Ten thousand years ago, an unspecified apocalypse nearly destroyed humanity, but a man referred to as the Emperor saved—and possibly revived—humanity with necromantic magic. This event is known as the Resurrection, and the Emperor has become God to humanity. The Emperor accomplished this feat with eight powerful Lyctors, immortal necromancers with companions called cavaliers. The secret of how to become a Lyctor is kept hidden.

After being saved in the Resurrection, humanity was divided into eight Houses on eight separate planets in the Dominicus system, each corresponding to one of the original Lyctors. Each House’s culture reflects the values of their corresponding Lyctor. An additional House, the First House, comprises solely the Emperor and his Lyctors. All houses are governed in a feudal system: There is a ruling family whose heir is attended by a cavalier, a life-long bodyguard. The relationship is modeled on the first Lyctors and their cavaliers who sacrificed themselves.

Gideon Nav, an 18-year-old swordswoman, attempts to escape the Ninth House, where she is enslaved, via an illegal shuttle. The Ninth House is nestled in a large crack on a desolate planet far from the sun and consists of terraced farms tended by skeletons with decrepit architecture reminiscent of cathedrals.

Crux, the marshal of the Ninth House, and Aiglamene, Gideon’s mentor, try to convince Gideon to come to Drearburh, the central keep, for a big announcement. Gideon refuses, and Harrowhark Nonagesimus, the 17-year-old heir of the Ninth House, offers to free Gideon if Gideon comes to Drearburh. Gideon refuses again, so Harrow challenges her to a fight; if she wins, Gideon must come to Drearburh, and if Gideon wins, she will be free. Harrow, a necromancer, rigs the battlefield with shards of bone that allow her to summon skeletons. She defeats Gideon and drags her to Drearburh. There, Harrow reads a letter from the Emperor. The Emperor requires the necromancer heirs of each House and their cavaliers to attend Canaan House, the “Temple” of the First House. The Emperor needs more Lyctors for his war since most of the original eight Lyctors have died. Harrow wishes to answer the call, but her official cavalier, Ortus Nigenad, doesn’t want to go for fear of death. He and his mother flee and steal the shuttle Gideon planned to use to escape. Harrow says that if Gideon replaces Ortus as her cavalier, Gideon can have whatever she wants when Harrow becomes a Lyctor. After three months of intense training, they leave for Canaan House. Gideon takes vows of silence to hide that she has not been raised as a cavalier since birth.

At Canaan House, the necromancers and cavaliers of the Second House through the Eighth House are introduced. Gideon quickly becomes interested in Dulcinea of the Seventh House, who is actually the Lyctor Cytherea the First in disguise. The Third House has sent two necromancers, twin sisters, breaking convention and House law. The head priest of Canaan House, named Teacher, says that there is only one rule: They must have permission to open locked doors because contacting the outside world is impossible until the trials are done. Teacher gives each cavalier a key ring and then allows everyone to explore Canaan House.

At Canaan House, Gideon reckons with her fraught past with Harrow and confronts her guilt over the death of Harrow’s parents. Harrow’s parents died by suicide when Gideon informed them that Harrow broke the Ninth House’s sacred rule by opening the Locked Tomb, a grave that the Emperor created the Ninth House to guard. Harrow is deeply self-loathing because her parents murdered every other child in Drearburh with a disease (except Gideon, who was miraculously immune) to make Harrow the perfect necromancer. Harrow takes out her anguish on Gideon, believing herself to be an abomination. Gideon and Harrow learn about an ancient laboratory under Canaan House, which holds “theorem chambers” that must be solved in order to access more keys for the cavalier’s keyring. Each chamber leads to a hidden room that belonged to one of the original Lyctors. Every Lyctor room contains a written theorem, on which the test to access it is based: Each of these theorems is a small part of the puzzle to becoming a Lyctor. The first theorem chamber is Harrow’s clue that she needs Gideon for the process, as it is impossible for her to solve without her cavalier.

Cytherea the First, still disguised, murders the Fifth House representatives, and cremates the original Dulcinea and her cavalier, creating paranoia among the Houses. As the heirs and cavaliers duel with one another for Lyctor door keys, alliances are made, and more houses are picked off. Gideon and Harrow align with the Sixth House, as Palamedes, the Sixth’s necromancer, is Harrow’s rival in intellect. Gideon and the Fourth House representatives look for the Seventh’s missing cavalier. The trio encounters a horrific bone construct that murders Isaac, the Fourth House necromancer. Gideon saves the Fourth cavalier, Jeannemary, only for Jeannemary to be murdered with bone spears when Gideon dozes off.

Terror pushes Gideon and Harrow to reconcile, and as their intimacy grows, nebulous romantic feelings develop between them. Before these feelings can blossom, the Second House pair is slain by Teacher when they attempt to contact the outside world. Teacher is also killed in the process. The key to Lyctorhood is solved by the Third House: A necromancer must absorb their cavalier’s soul and use it as an “eternal battery” to fuel their own immortality and godlike powers. The Third necromancer, Ianthe Tridentarius, becomes a Lyctor by absorbing her unwilling cavalier and murders the Eighth House representatives.

Cytherea is exposed by Palamedes, who turns himself into a necromantic bomb in an attempt to kill her. Cytherea lives and attacks the remaining survivors: Gideon, Harrow, and Palamedes’s cavalier, Camilla. Since the trio’s powers are inferior to the Lyctor’s, Ianthe attacks Cytherea, buying them some time. When Cytherea defeats Ianthe, Gideon decides to kill herself so Harrow can absorb her and become a Lyctor to defeat Cytherea. Harrow defeats Cytherea and passes out. She wakes up in a sickroom on the Emperor’s flagship with the Emperor himself watching over her. Heartbroken, she begs for Gideon back. When the Emperor says he cannot do that, she pledges herself as his Lyctor, unable to bear her own reflection.