Sixteen year old Cinder is a cyborg who has a mechanic booth in the city’s marketplace. One day, as she is fitting herself with her new metal foot, Prince Kai comes to her, asking her to fix his android, Nainsi. Cinder likes Kai and agrees, but she hides her true identity from him, for fear of being shunned. A short while later, an outbreak of the deadly plague, letumosis, spreads throughout the marketplace.
When Cinder arrives home, she finds her stepmother and stepsisters getting fitted for the ball, and Cinder says she would like to go as well. Her horrible stepmother says that they cannot afford a gown for her and repair their hover, so Cinder decides to go to the junkyard to see what she can find to fix the hover herself. Her younger stepsister, Peony, goes with her, and while they are there Peony breaks out in rashes; she has contracted letumosis and is taken away to quarantine. Cinder is tested and is negative for the disease. Angry at Cinder for potentially exposing Peony, her stepmother, Adri, volunteers Cinder for letumosis research and Cinder is taken away against her will.
At the palace, Prince Kai’s father, Emperor Rikan, has also contracted letumosis, and the prince must take over his reign. His immediate responsibility is to come up with a peace negotiation with Queen Levana of Luna, the colony of the moon. The queen and her people, the Lunas, are known for their manipulative powers to make themselves look good and more powerful. The queen wants to marry Kai to form an alliance, but Kai wants to find the queen’s missing niece, Princess Selene. No one knows if the princess died in a fire or if she fled Luna and is living on Earth.
The next day, the emperor dies, and Cinder visits Peony in quarantine, promising she will do all she can to save her. During research, Dr. Erland discovers that Cinder is immune to the disease and could therefore be useful to helping find a cure. When Cinder returns to the palace’s research center, she runs into Kai and gives her condolences. Kai invites her to the ball as his personal guest, but she declines. At the lab, Dr. Erland reveals that the reason Cinder is immune is because she is, in fact, a Lunar without the Lunar powers of manipulation, and is therefore someone whom the queen will try to kill if discovered.
When Levana arrives to continue peace talk negotiations, she brainwashes the disapproving crowd. She tells Kai that she has an antidote for the deadly disease, and is willing to share more than the one dose she gives him for a price. Meanwhile, Cinder discovers an unusual communication chip in Kai’s android and becomes suspicious. When she returns Nainsi to the prince, they head to Dr. Erland’s together, and Kai gives him the vial from Levana and asks if the doctor can replicate it. Cinder learns some information about her past and visits Peony with half of the vial of antidote. Peony dies before she can drink it, and Cinder gives it to Sunto, a young boy in quarantine. When she returns home, Adri takes away her new prosthetic foot to sell, as she has already sold off the parts of Iko, Cinder’s beloved android.
Kai gives Cinder a special gift on the day of the ball, and urges her once again to join him. Kai tells her that he plans to marry Queen Levana in order to protect his country. While watching the coronation, Cinder learns that the suspicious chip in Nainsi was implanted to spy on the prince, and that Levana plans to kill Kai after they marry and produce an heir to the throne. Cinder realizes that she must tell Kai. She throws on her old foot and Peony’s dress and drives the gasoline car she found at the junkyard to the ball. When she arrives, she warns Kai about Levana’s plan, but Kai still says he will go through with the marriage.
When Kai and Cinder kiss, Levana gets angry and tells everyone that Cinder is a Lunar fugitive and must be taken away. Under Levana’s spell, Cinder takes a gun from a nearby guard and points it at her own head. Cinder is able to break free from the spell and shoots the chandelier instead, causing the crown to run out of the ballroom. After Cinder drops the gun and runs, her foot is severed and she falls to the ground in pain. Kai comes to her aid and discovers she’s a cyborg and has been hiding it from her. Disgusted and all trust lost, Kai has Cinder sent to prison to await transport back to Luna. Dr. Erland visits Cinder and reveals to her that she is, in fact, Princess Selene, and that she must escape and meet him in Africa. They must find a way to stop the war and gain control of Luna from Levana.
As the book ends at this point, readers are left wondering, what will happen to Cinder? What will happen to Kai and the Eastern Commonwealth? Cinder’s story, along with the stories of others, continues over a series of novels in The Lunar Chronicles.