Nemesis is a Diabolic, a genetically engineered humanoid bred to protect the elite and wealthy in the galaxy. Diabloics are bonded to one person once they have proven themselves to be strong and deadly. They will defend that person against any threat, even if it means they sacrifice their own lives.
Nemesis is bought by Senator von Impyrean to protect his daughter, Sidonia. Once Nemesis is bonded to Sidonia, she never leaves the girl’s side. The two grow up together, and though Sidonia insists that Nemesis is more human than a mindless weapon, Nemesis refuses to believe her.
An order arrives from the Emperor to destroy all Diabolics because some have gone rogue. They have killed not only to protect their masters, but also to advance their political and social standing. Sidonia pleads with her father to spare Nemesis. Since her father bears only animosity toward the Emperor, he agrees. When an Inquisitor is sent to make sure the order has been carried out, he is shown the body of a dead servant instead.
Senator von Impyrean also opposes the Emperor’s ban on technology and the pursuit of knowledge. The Emperor and the Helionics, those in the ruling class who believe in a religion of the stars, do not want people to gain knowledge because it may give them more power. Instead, they are willing to let planets, space stations and starships decay rather than seek to fix them or create new ones.
When other senators who agree with Impyrean become too vocal, the Emperor issues a new command. All the ruling families must send their heirs to the Chrysanthemum, the Emperor’s Imperial court. Sidonia’s parents believe it is to hold the children hostage so their parents will not rebel. They fear Sidonia will be killed for her father’s disobedience. They concoct a plan to transform Nemesis’s body and features so she appears more human and send her in Sidonia’s stead.
As her body is transformed, Nemesis is also taught human etiquette and how to fake human emotions. When a distraught Sidonia admits their ruse to their etiquette instructor, Nemesis snaps the woman’s neck. She cannot risk anyone telling the Emperor the truth, or Sidonia suffering his wrath.
Despite being horrified at her actions, Sidonia confesses she loves Nemesis. Nemesis claims she can’t return the feelings because of the way she was created. Sidonia begs her to do everything she can to stay alive, for if Nemesis dies, Sidonia will kill herself. Nemesis agrees, as she must do everything she can to protect the young woman.
Nemesis arrives at the Chrysanthemum and is immediately greeted by the barbarity of the Emperor and the immorality of his relatives. The Emperor demands one of her servants skin herself alive as they watch. Desperate to react as Sidonia would, Nemesis pretends to faint.
Later, the Emperor’s niece invites Nemesis to her and her husband’s quarters for a salt bath. Another girl, Neveni, cryptically warns Nemesis not to drink their wine. Since she is a Diabolic, drugs do not affect her, but she is horrified to learn that the couple planned on raping her once she was drugged.
Furious, she forces the pair to drink an overdose of their own drug, and it renders them comatose. As Nemesis seeks to hide what she has done, Neveni jumps out with a knife, ready to strike. She had come to save Nemesis, as she had experienced the same assault. Neveni offers to use her skill with computers to wipe away any evidence of Nemesis’ guilt. The two girls become friends.
Nemesis and the other heirs are called to a special Convocation. The Emperor addresses the crowd and tells of his anger that some in the galaxy do not wish to adhere to his laws. There are senators and viceroys who disagree with his rule against studying technology. For that reason, they and their families have been killed. The surviving heirs will have their parents’ titles and will be expected to abide by the Emperor’s decrees.
Nemesis refuses to believe Sidonia has been killed. For days she tries to contact the Impyrean stronghold, but no one answers. Finally realizing Sidonia is dead, Nemesis strikes out to kill the Emperor.
Nemesis is stopped by one of his Diabolics. Before the creature can kill Nemesis, Tyrus, the Emperor’s insane nephew and heir, interrupts them. When he discovers Nemesis wants to kill his uncle, Tyrus kills the Diabolic and takes the wounded Nemesis back to his quarters.
As Nemesis heals, she comes to realize Tyrus is not insane. He has played the madman in order to keep his uncle from killing him, as his parents and most of his family were assassinated. Tyrus convinces Nemesis to keep pretending to be Sidonia. They will act like a couple in love, and Tyrus will slowly convince the Court that her love is calming his madness.
Once he has proven himself to be sane, they will kill the Emperor, and Tyrus will rule. He promises he wants to help those like the Impyreans, people who long to study the sciences and to learn how they can improve their lives and flourish.
As Tyrus and Nemesis work together, she finds herself in the confusing position of developing feelings for him. She used to argue with Sidonia that Diabolics could never feel emotions like real humans, but Nemesis is attracted to Tyrus and the two share several intimate moments.
The Emperor sends Tyrus and Nemesis on a diplomatic mission to Neveni’s planet to calm unrest. He wants Tyrus to use a biochemical weapon to kill most of the inhabitants. Nemesis helps to thwart an assassination attempt on Tyrus’ life and he, in turn, is able to convince the people that once he is Emperor, he will work to restore their world and others. They agree to let them go. Tyrus and Nemesis return to the Chrysanthemum to a hero’s welcome, although the Emperor is not pleased that his heir did not destroy Neveni’s planet.
As Nemesis’ feelings for Tyrus continue to grow, she is shocked to learn that Sidonia is alive. She snuck away from home to visit Nemesis before the Emperor’s attack. Sidonia can’t take her place in the Senate because it will reveal their deception to the Emperor and would mean certain death for both her and Nemesis.
Eventually Nemesis tells Tyrus that Sidonia is alive so he can help them both live. After speaking to Tyrus in private, Sidonia confesses her love to Nemesis. She tells her that Tyrus is not convinced they can transition Sidonia back to being a senator even if he is successful in taking his uncle’s throne. Both she and Tyrus are committed to protecting Nemesis’ life above all else.
Tyrus’ grandmother, Cygna, suspects that her grandson’s madness has been a ruse. She attempts to coerce Nemesis into spying on him for her. When she refuses, Cygna becomes her enemy. After an attempt on his and Nemesis’ lives, Tyrus pretends to switch his affections from Nemesis to the Impryeans’ sworn enemy, a girl named Elantra. He announces his engagement to her, and she demands that Nemesis attend her on the morning of the betrothal ceremony. Since Nemesis does not know the elaborate patterns she must paint on Elantra’s skin, Sidonia pretends to be her servant and gives directions.
When it comes time for Nemesis to anoint herself with special oil, Sidonia insists on doing it for her. The oil is poisoned, and Sidonia quickly succumbs to the effects. Nemesis, however, does not. She rips out Elantra’s heart just before Cygna arrives.
Cygna spins a tale that indicts Tyrus with the plotting and the poisoning. He knew the poison would not affect Nemesis but kill Sidonia. He knew Nemesis would kill Elantra in retaliation. Now he is free to marry Nemesis, and Sidonia would never be revealed as the true senator. When Nemesis accuses both Cygna and Tyrus of conspiring against the Emperor, he orders his Diabolics to kill his mother, but they refuse. They are actually bonded to Cygna.
Cygna orders them to kill the Emperor and then demands Tyrus blame Nemesis if he wants to be the new Emperor. At his coronation ceremony, Nemesis will be the fire sacrifice. Tyrus agrees, and Nemesis is imprisoned to await her death at the ceremony.
When the day arrives, Nemesis is escorted under guard to the ceremony. Before she can be sacrificed, Tyrus stands and reminds his grandmother that she once made a rash vow, that if he were ever to be crowned Emperor, she herself would be the fire sacrifice. As her Diabolics try to save her, guards attack them.
Nemesis realizes that Tyrus has managed to sneak in senators and viceroys, who have pledged their allegiance to him rather than his grandmother. Cygna admits to killing her son, the Emperor. She also confesses that she, not Tyrus, conspired to have Sidonia and Elantra killed. With her final words, Cygna warns the people that Tyrus is the cleverest fiend of all her relatives.
Tyrus begs Nemesis to marry him and become his Empress, even though many believe Diabolics are monsters. She agrees, but knows she will have to be careful. She wants to believe that Tyrus is noble and true, but she must never let her guard down, in case he is the tyrant of Cygna’s warning.