Glass Sword picks up right where Red Queen left off, after Mare and Cal have escaped execution. Rescued by the Scarlet Guard, Mare and Cal now find themselves on an underground train with Farley, Kilorn, and other Guardsmen. Astonishingly, Mare’s older brother Shade is alive and here too, working with the Guard. Shade is like Mare—he has red blood and a special ability, which makes him both Red and Silver. While Mare can create electricity and control lightning, Shade can teleport. In a world sharply divided between powerful Silvers and lowly Reds, Mare’s and Shade’s abilities make them different from and stronger than everybody else. They’re newbloods, and Mare knows there are even more people like them. She intends to find the other newbloods before Maven can hunt them down and kill them.
Mare, Cal, and the other Scarlet Guard members arrive in the ruined city of Naercey. Before long, Maven’s forces attack. Maven himself appears and threatens Mare, but Mare escapes from him. She and Cal follow Farley into a submarine, which takes them and the rest of the Scarlet Guard out of danger.
On the submarine, Mare struggles with internal and interpersonal problems. She doesn’t know who she is anymore. She fears that the traumatic experiences she’s undergone have changed her beyond recognition. After suffering Maven’s betrayal, she’s become distrustful of everyone. Moreover, Mare is hurt that most Guardsmen seem to be afraid of her and dislike Cal automatically because he’s Silver. These problems only intensify after the submarine reaches its destination: the island of Tuck, where a Scarlet Guard base is hidden. Mare grows upset when the Colonel—a Scarlet Guard leader from the Lakelands—imprisons Cal, despite her objections that Cal is on their side. Additionally, Mare finds it difficult to reunite with her family, who now live on Tuck as refugees. They don’t know what Mare has been through or who she’s become. Mare worries they’d fear and hate her if they discovered everything she’s done to survive.
Mare feels unwelcome on Tuck due to the Colonel’s prejudice against anyone who isn’t a “true Red.” When she and Farley mention their plan to rescue all the newbloods on Julian’s list, the Colonel harshly objects. He considers newbloods “freaks” and Mare’s plan a waste of time. Angered by the Colonel, Mare formulates her own scheme to break into the underwater barracks where Cal has been imprisoned. Kilorn helps her, but when they reach Cal’s cell, Kilorn pushes Mare inside and locks her up with the exiled prince. Mare is shocked and furious to be betrayed by her best friend. However, Kilorn reveals that he was helping Mare and Cal all along. Kilorn, Farley, and Shade work together to trick the Colonel, break Cal and Mare out of their cell, and escape Tuck on a stolen Blackrun jet.
Once in the air, Mare, Farley, Kilorn, Shade, and Cal prepare to locate all the newbloods on Julian’s list. First, they fly to the village of Coraunt, where they find Nix Marsten. Mare tells Nix that he’s a newblood and gives Nix the choice to join her in her fight against Maven or not. Nix is loath to work with Cal, whom he blames for his daughters’ deaths, but he eventually agrees to join Mare.
Next, Mare and her team land near Harbor Bay, where they plan to recruit Ada Wallace and Wolliver Galt. Outside the city, Mare, Cal, Farley, and Shade meet Crance and the Mariners—some of Farley’s friends—who take them through underground tunnels into Harbor Bay. However, it soon becomes clear that Crance is leading them into a trap, and fighting breaks out. At the last moment, Crance resolves to help Mare rather than turn her over to Maven. As parts of the tunnel collapse, Crance leads them to safety. Mare and the others break into Harbor Bay’s Security Center to obtain detailed records of more newbloods on Julian’s list. They escape from Silver officers and reach Wolliver Galt’s residence in the city, only to find that Wolliver is already dead. Then, Maven and other Silver soldiers appear and attack. Maven tortures Mare with a sounder device, which turns her own lightning power against her.
Four days later, in a Scarlet Guard safe house called the Notch, Mare wakes up with new scars and an M-shaped brand burned into her chest by Maven. While Mare has been unconscious, Crance has recruited Ada Wallace, and the rest of Mare’s team have been searching for more newbloods. Alone with Cal, Mare struggles with her competing desires to be close to Cal and to distance herself from him. She believes she needs to be a weapon in order to destroy Maven and Queen Elara, but she fears that she’s “a sword made of glass” that is “beginning to shatter.”
Over the next few weeks, Mare travels all over Norta on recruitment missions, rescuing more newbloods from Maven’s grasp. Still, Maven torments her by leaving notes for her. Mare won’t give in to Maven, but she secretly collects his notes because she misses the person he once pretended to be. Meanwhile, at the Notch, Cal and Mare train with the newbloods to help them become better fighters and learn to control their abilities.
On the next mission, Mare, Cal, Shade, and Farley travel through an abandoned mining village called Rosen. There, they encounter Jon, a mysterious newblood who can see the future. Jon refuses to join Mare’s team, but he instructs them to raid Corros Prison. Corros is a facility where Maven has imprisoned newbloods and disloyal Silvers, including Julian and Sara Skonos. According to Jon, Queen Elara will turn the newbloods into her own weapons if Mare and the others don’t stop her. Mare decides to trust in Jon’s predictions and free the prisoners from Corros. When Jon tells her to go to Little Sword Lake first to find something important, she follows his advice.
At Little Sword Lake, Mare finds not something but someone—Cameron Cole, a newblood who has just escaped Corros Prison herself. Cameron wants nothing to do with Mare. Instead, she intends to head straight to the Choke to save her twin brother, who’s been conscripted into the “Little Legion” along with thousands of other Red children. Maven has ordered this legion to march into the Choke—a sure death sentence. However, Mare forces Cameron to join her team and help them break into Corros. Cameron accuses Mare of favoring “special” newbloods over “ordinary” Reds.
After three days of preparation, Mare, Cal, Shade, Kilorn, Farley, Cameron, and seven other newbloods successfully infiltrate Corros Prison and begin freeing prisoners. A battle erupts as some of the released prisoners join the fight against their Silver jailers. Mare kills several Silver officers mercilessly. She and Cal rescue Julian and Sara from their cells, but then Mare is cornered by Queen Elara. Shade rescues Mare, teleporting her away. Outside the prison, Ara Iral saves Mare’s life and fights against Ptolemus Samos. But Ptolemus kills Ara, and—in an attempt to pierce Mare with a metal blade—kills Shade as well.
The next thing Mare knows, she’s on a jet flying away from Corros with the surviving members of her team and hundreds of freed prisoners. Beside her are the bodies of Shade and Queen Elara, whom Mare killed with her own lightning. Devastated by Shade’s death, Farley accuses Mare of knowingly sacrificing her own brother in Corros just to acquire more newblood followers. Cal is also disturbed by Mare’s recent cold-hearted behavior. Instead of defending herself, Mare simply explains her newest plan. First, she’ll expose Maven’s crimes to the entire kingdom in a public broadcast to incite civil war. Then, she’ll go to the Choke to save the Little Legion.
After landing in Tuck, Mare presents the Colonel with Elara’s corpse as a “gift.” Although the Colonel is angry with her for bringing Silvers and more newbloods into his base, he helps her record her broadcast and takes her to meet Rash and Tahir. These twin men are ambassadors from the Free Republic of Montfort, a nation where newbloods are safe to live openly. Rash and Tahir have allied with the Scarlet Guard and want to make a deal with Mare to liberate newbloods. However, Mare refuses their deal, because she wants to focus now on helping Reds in Norta. She tells the Colonel her plan to go to the Choke, and he agrees to help her save the Little Legion. While preparations begin for this next mission, Julian finds Mare and warns her that she could become evil if she keeps walking down the dark path she’s currently on. Mare promises she’ll do better. She visits her grieving family, and her older brothers Bree and Tramy volunteer to come with Mare to the Choke. After Shade’s funeral, Mare’s team departs from Tuck on the Blackrun. However, the jet explodes, and Mare and her team fall into a cage secured by Maven’s Silver soldiers. To save everyone she loves, Mare surrenders to Maven in exchange for her allies’ freedom.
Days later, Mare’s Silver captors parade her like a trophy to the royal palace in Norta’s capital city. Jon is there, watching as Mare is brought before Maven. Maven puts a collar around Mare’s neck and forces her to kneel to him.