17-year-old Mare Barrow is the fourth of five children born to ordinary Red-blooded parents. Her three older brothers have already been conscripted into the army, pawns in a war started long ago by the ruling, almost immortal, Silver-blooded nobility. Mare knows she will be shipped off to war as soon as she reaches 18, as she has no skills the Silvers deem exceptional. Her younger sister, Gisa, will be able to provide for their parents, as she is a talented seamstress.
When Mare’s best friend, Kilorn, loses his apprenticeship, Mare vows to help him escape the army. Gisa smuggles Mare into a protected city, run by Silvers, in order for Mare to steal the money she needs to pay for Kilorn’s freedom. Before she can act on her plan, however, the city is thrown into chaos as the Silvers learn of a terrorist attack in the capital. The culprits are a group of Reds known as the Scarlet Guard. As Mare and her sister flee the city, Gisa is caught trying to pick the pocket of a Silver in order to get the money Mare needs. Gisa’s hand is crushed as punishment for her crime, thus assuring the future starvation of their family.
Upset at having caused the ruin of her family, Mare goes out for a walk at night. She meets a young man outside a tavern who stops her from stealing from the other patrons, but gives her coins to help her family. Mare tells him all that has happened, expecting nothing but friendly concern in return. In the morning, she is escorted by guard to the royal palace and given a position as a servant. That evening, while serving at a function for the royal family, she discovers the man she spoke with the night before is the crown prince, Cal.
The ruling Silver families have come to the palace to present their daughters for consideration to become his bride, as well as the wife of his younger stepbrother, Prince Maven. The hopeful contenders take turns displaying their superhuman abilities. All Silvers possess some kind of power. Some can manipulate flame; others can control water. Still others can teleport, make things grow, or read minds. When Evangeline, a Silver with the ability to control metal, manipulates the spectators’ boxes so that Mare falls into the arena, everyone expects her to be killed by the electrical currents surrounding it. Instead, Mare takes the energy within her body and uses it to escape Evangeline’s attack. Mare tries to flee the castle, but is stopped by Prince Cal.
Queen Elara, Prince Cal’s stepmother, uses her mind-reading capabilities to search Mare’s memories. When it is discovered that she is only a Red, King Tiberias decides to hide that knowledge from the world. He can’t kill her, as everyone saw her survive and assumes she has Silver blood, but he can’t let it be known she does not, or the commoners will revolt.
Tiberias and his family decide to claim that Mare is the daughter of slain Silver-blooded parents killed in combat. A Red soldier found her and raised her as his own daughter. Now that she has been found, she will wed Prince Maven as a reward for her parents’ sacrifice to the kingdom. Mare agrees to the deception as long as the king brings her brothers home from the war and provides for her family.
Mare must learn how to act like a Silver lady of nobility. She suffers through long hours of protocol training and luncheons with other nobles. The highlight of her training comes from Julian. He is Prince Cal’s uncle, the brother to the first queen. Julian is supposed to teach her history, but he also teaches her how to manipulate her newfound powers. When her powers seem to become too great for her to control, she is brought in to train with other Silver-blood nobility. Evangeline, now Prince Cal’s fiancee, challenges Mare to a duel and wounds her. Before the others can see her red blood, Prince Maven hurries her to a healer they know will keep her secret.
Although betrothed to Prince Maven—the quieter of the two brothers—Mare finds herself drawn to Prince Cal, who arranges for her to secretly visit her family. She learns that her brothers did get released from the army, but that the youngest, Shade, was killed in action before the order came. Mare seeks out the leader of the Scarlet Guard, Farley, in order to declare her allegiance to them. She is surprised to find that Prince Maven, too, wishes to help in the revolution against the ruling class.
Together, they help the Scarlet Guard enter the palace and assassinate three top officials. Unfortunately, several members of the Guard are captured during the fight. Among them are Mare’s friend Kilorn. Mare begs Julian to use his ability to manipulate minds to coerce various palace guards into helping Kilorn and the other Scarlet Guard members to escape. During the attempt, a guard shoots Mare. The blood she spills near the jail will be analyzed. The truth of Mare’s identity will be revealed. Prince Maven vows to protect her.
In light of the rising tensions, King Tiberius and his family return to the capital. Along the way, Mare sees more cities populated by Reds, who have been forced to serve the Silvers as little more than slaves. Mare is forced to give a speech denouncing the Scarlet Guard and extoling the virtues of their Silver-blooded rulers. Farley finds her and takes both Mare and Prince Maven on a tour of the Scarlet Guard’s secret network of tunnels.
The Guard has the ability to travel quickly and undetected throughout much of the land. Mare gives a list of names to Farley—other Reds who have even stronger abilities than the Silvers. Prince Maven convinces Farley and the Scarlet Guard to arrange a coup against the king. He believes that Prince Cal’s affection for Mare will bring his brother and the troops he commands to the rebels’ side.
The rebels blow up a bridge in the center of the city. When Prince Cal and his troops come to investigate, Mare tries to convince him to join the Scarlet Guard and overthrow the current government. Although torn by his feelings for Mare, Prince Cal refuses and has her and Prince Maven arrested.
When they are brought before the king to be tried, Queen Elara uses her powers of mind control to force Prince Cal to kill his father. Mare realizes she has been a pawn in a power struggle between the queen’s family and King Tiberius’ family. Queen Elara and Prince Maven betray Prince Cal and Mare. They testify that Prince Cal willingly killed the king.
Prince Cal and Mare are sent to the arena to be killed. Instead, after a great battle, Mare and Prince Cal manage to overpower, or kill, their attackers. The Scarlet Guard helps the two escape capture by Prince Maven and his guards. Mare falls unconscious.
When Mare awakens, she discovers her brother Shade is not dead. He, too, has abilities and could not be killed by the Silvers. Together with Kilorn and Farley, they vow to find the other mutant Reds before Prince Maven has them killed. The Guard holds Prince Cal prisoner, unwilling to believe he is ready to join their fight against Prince Maven, but Mare knows the truth. She and Prince Cal now have the same goal: to kill the one who betrayed them.