It has been seven hundred years since mankind colonized other planets. The powerful ruling class of humans has installed a rigid, color-based social hierarchy where the physically superior Golds at the top rule with an iron fist. Sixteen-year-old Darrow is a Red, a class of workers who toil beneath the surface of Mars mining helium-3 to terraform the planet and make it habitable. He and his wife Eo are captured after entering a forbidden area and are arrested. While she is publicly whipped for her crime, Eo sings a forbidden folk tune as a protest against their virtual enslavement. She is subsequently hanged on the orders of Mars' ArchGovernor Nero au Augustus. Darrow cuts down and buries his wife's body, a crime for which he is also hanged. However, Darrow awakes to find that he has been drugged and delivered into the hands of the Sons of Ares, a terrorist group of Reds who fight against the oppression of the "low Colors". They have adopted the video of Eo's song and execution as a rallying vehicle for their cause. Darrow joins the Sons when he learns that Mars was already terraformed centuries before and that the Reds have been tricked into perpetual servitude and subjugation. Darrow is conscripted to impersonate a Gold and infiltrate the Society to bring it down from within. He is physically transformed by Mickey, a Violet "carver", who gives him the abilities and appearance of a Gold through painful surgeries, treatments and implants. Using a fabricated identity and succeeding at a sort of placement test, Darrow is accepted into the Golds' elite Institute, where he befriends the charismatic Cassius au Bellona and alienates the arrogant Antonia au Severus. Darrow is selected for House Mars by its gruff Proctor, Fitchner. To continue to the next stage, Darrow must complete the Passage, a test in which the 100 newly chosen students in each of the twelve Houses are paired with another house member and tasked to kill each other as a means to eliminate the weaker half. Darrow is forced to murder Cassius' brother Julian to survive, but Cassius can only guess who may have killed him. Each House is assigned a fortress and a scepter, called a standard, to defend within the Institute's confines, with the goal of warring with each other until one House enslaves all others with the standard. Mars fractures into factions: one led jointly by Darrow and Cassius, one by Antonia, one by Titus au Ladros, and the antisocial Sevro going off on his own. To neutralize the violent Titus, who has been raping the female slaves left in Mars' fortress, Darrow manipulates House Minerva, led by the young woman he met briefly upon his arrival at the Institute and whom he has dubbed "Mustang". Mustang and her troops take the Mars fortress and imprison Titus. Sevro helps Darrow escape and capture Minerva's standard, which he trades to reclaim Mars' castle. Darrow takes over as the Primus (leader) of Mars, and Sevro and his group of "Howlers" declare their loyalty to him. Darrow realizes from the captive Titus' manner of speech that he is a fellow Red impersonating a Gold. To maintain his cover, Darrow allows Titus to be executed. Darrow captures Minerva's fortress and defeats their strongman Pax au Telemanus. Before she escapes, Mustang reveals the existence of "the Jackal", the leader of House Pluto who is terrorizing other Houses. Antonia and some of Titus' former followers attempt to overthrow Darrow, but he manages to thwart their plan. Lilath, a messenger from the Jackal, secretly reveals to Cassius that Darrow killed Julian. Cassius challenges Darrow to a duel outside Mars' fortress, severely wounding him and leaving him to die in the snow. Darrow is rescued and nursed back to health by Mustang, who still has Minerva's standard despite losing their fortress. They begin to develop romantic feelings for each other as they flee to avoid discovery by Cassius, now Primus of House Mars. Conquered students are systematically "enslaved" by other Houses, forced by their honor to serve their conquerors. Darrow and Mustang begin to amass an army by recruiting many Oathbreakers, the wandering slaves who have chosen to disobey orders, with Minerva's standard. Darrow prefers that his captured foes swear their allegiance and join him, rather than serve him. Learning from his previous mistakes, he frees these slaves and takes responsibility for their actions to gain their allegiance. He gains the loyalty of the duplicitous Tactus au Rath when he accepts physical punishment on himself after administering the same to Tactus for unruly behavior. Sevro, who has led his team of Howlers to escape from Cassius and has now lost an eye, meets up with Darrow again to join forces. Darrow takes the fortresses of Houses Ceres, Apollo, and Jupiter, enslaving their members until the prisoners prove their loyalty to him. Fitchner reveals to Darrow that the other Proctors have been conspiring to assist the Jackal, who is actually Adrius, the son of ArchGovernor Augustus. Darrow exposes Lucian, a prisoner taken during the surrender of house Jupiter, as the Jackal after impaling his hand and offering him the opportunity to free himself by cutting it off. The Jackal slices off his own hand to escape and when it becomes clear that Darrow has decided not to let him leave, the Jackal murders Pax and escapes with the assistance of Proctor Apollo, the main proctor assisting the Jackal. Enraged by the Proctors' deliberate efforts to hinder his victory, Darrow slays Proctor Apollo, and his army storms Mount Olympus, the floating palace of the Proctors using the flying boots gained from proctor Apollo. With the remaining Proctors subdued with the help of Sevro and the Howlers, Darrow sends Mustang to capture the Jackal, only to find out from Fitchner that she is Virginia au Augustus, the Jackal's twin sister. Darrow expects a betrayal, but she returns to deliver her captive brother, and Darrow wins the exercise. Before he departs, Cassius promises Darrow revenge. With his victory, Darrow is given his choice of a patron to sponsor his future. He accepts the hated ArchGovernor Augustus' offer to serve as one of his lancers, knowing that the powerful man will offer him the greatest opportunities to acquire the power he needs to destroy the Golds.