Requiem is told from the perspectives of both Lena and Hana, Lena's best friend from Portland, Maine, where the government has declared love, or Delirium, a disease. Lena is still living in the Wilds, an unregulated area outside of the city. She is now an active member of the resistance, which is fighting against the government and its mandated cure of Delirium. Meanwhile, Hana is cured of Delirium and living a privileged and safe life in Portland. Hana is also engaged to marry the city's young mayor, Fred.
The story starts a few days after Lena rescues Julian from execution and learns that Alex is still alive. Alex, the rebel boy Lena fell in love with in the first book, is angry at Lena for her relationship with Julian, so he doesn't speak to her for a few days. Lena is torn between the two men and avoids being affectionate to Julian while she sorts through her feelings. Julian is hurt by how Lena's behavior has changed toward him, but he is patient and gentle with her. He doesn't expect Lena to return to the way things were between them. When Julian tells Lena that he loves her, she tells him to give her time.
When Alex finally does speak to Lena, he tells her that he was tortured after he was captured by the government. The hope that he would see her again was the only thing that kept him going. But that is all in the past; he does not love her, and he never loved her.
Lena is reunited with her group of rebels, including Raven and Tack. They decide to join with other resistance groups and fight the government instead of hiding from it.
Meanwhile, Hana is concerned about feelings of worry that she has for Lena and her family, feelings that should have been wiped away when she was cured. While Hana is out with her mother, their driver nearly hits a young girl running in the streets. Hana recognizes the girl as Lena's cousin Jenny. Before Hana can say anything to her, Jenny runs away. A few days later, Hana visits Lena's old neighborhood. She learns that when Lena ran away to the Wilds, her family lost their jobs and home. They had to move to a rundown neighborhood called Deering Highlands, where other outcasts live. Hana rides there and finds Grace, Lena's youngest cousin. She gives her food.
While heading south, Lena and her group find a homestead they had stopped at months before. Government regulators have killed the owners and burned their bodies. The group is alarmed that regulators have come this far into the Wilds. A few nights later, regulators attack the group, and their camp is burned to the ground. There are deaths on both sides, and the group is forced to move without essential supplies.
At first, Hana is satisfied with her fiancé, Fred, and believes they are compatible, but as the wedding date draws near, she begins to see a different side of him. Fred tells her that the government has been too lax and has allowed too much freedom. As mayor, he plans to give electricity only to people who fully support the government.
Hungry and exhausted, Lena's group arrives at a larger rebel camp. They are disappointed to find a disorganized group of rebels living in filth and fighting each other for basic necessities. They join with one of the camp's leaders, Pippa, and learn that the government is sending an army to kill all rebels in a few days. Pippa urges Lena's group to move on and meet with some of the leaders in the resistance. They leave just before the soldiers and tanks move in and massacre the group.
Hana is curious as to what happened to Fred's first wife, Cassie. She asks Fred, and he becomes physically abusive, grabbing and pinching her while telling her that Cassie asked too many questions. After Fred has someone follow Hana, he learns she had been sneaking out on her bicycle. He confronts Hana, threatens her, tries to choke her and tells her that she can never return to the Deering Highlands neighborhood. Hana learns that Fred had Cassie committed to a mental institution, making it easy for him to divorce her. Hana secretly visits Cassie, who tells her that Fred tortured and killed animals as a child, slowly poisoned her over a series of months and arranged to have his father killed during riots so he could inherit his job as major.
Lena's small group of rebels goes to a hideout to wait for the resistance leaders to contact them. Exhausted, Lena goes to sleep and wakes up to find out that Julian has asked Alex to teach him how to fight. She runs outside to see the boys fighting. Julian is badly hurt. Lena is angry with Alex, who apologizes to her and leaves the camp. When the resistance leaders finally arrive, Lena recognizes one of them as her mother. Lena had long believed her mother was dead and only recently learned she was still alive. The leaders tell them that something big is about to happen in Portland, and although Lena initially refuses to return to the city, she agrees in the end. Lena's small group joins a larger group outside the barrier walls of Portland and wait to attack the city. Lena also discovers that bombs will be planted at several key buildings throughout Portland.
The morning of Hana and Fred's wedding is tense with the knowledge that rebels are outside the city planning an attack. Just before the couple is pronounced man and wife, sirens go off, signaling that rebels have breeched the city walls. Bodyguards take Hana back to Fred's house.
While the rebels are fighting at the wall, Lena runs to her old neighborhood in hopes of finding her cousin Grace. Hana's car almost collides with Lena, and Hana's bodyguards take Lena back to Fred's house. Knowing that Hana is cured, Lena does not know what to expect. Hana, noticing how thin Lena has become, gives her food. She also confesses that she was the one who told the regulators about Lena and Alex because she was jealous of their relationship. She was the reason Lena was caught and Alex was arrested and tortured. Angered by this confession, Lena slaps Hana in the face. This doesn't stop Hana from giving Lena her family's new address, and Lena in turn tells Hana that a bomb has been planted in Fred's house.
Hana helps Lena escape before Fred gets home, and Lena heads for her family's neighborhood, which has been set on fire by regulators. Lena finds Grace, who has been abandoned in a burning house by Lena's aunt and uncle, and takes her to safety.
Fred comes home, and Hana lies to him, telling him that Lena attacked her and escaped. Fred is angry and wants to leave the house so he can get control of the situation, which now involves rebels and uncured teens rioting in the streets. But Hana convinces him that he will be safer in the house. He agrees, and she slips out of the house and walks away before it explodes. She does not know what will happen to her, but she has a feeling she will be OK.
In the aftermath of the attack and riots, Lena sees Alex, who was in Portland as well. He admits he still loves her, and she tells him she loves him, too, but things are complicated for her. After Alex tells Lena that he won't run away again, he kisses her. They see people she knows tearing down the wall that separated Portland from the Wilds, and Lena and Grace join them. She does not know what the future holds for her, Alex and Julian, but she chooses to move forward.