Katniss, her sister Prim, and her friends Finnick and Gale all reluctantly adjust to a highly structured life in the underground District 13, which has been spearheading the rebellion in Panem. Feeling manipulated, Katniss eventually agrees to act as "the Mockingjay"—a poster child for the rebellion—but only on the condition that District 13's President Alma Coin vows to grant immunity to all of the past Hunger Games tributes, including Katniss's friend Peeta Mellark and Finnick's lover Annie Cresta, and to reserve for Katniss the right to personally kill Panem's President Snow once he is captured.
Katniss and the rebels learn that Peeta is alive and is being tortured at the Capitol in an attempt to demoralize and control Katniss. When a rescue team succeeds in extracting him, they discover that Peeta has been "hijacked"—a brutal form of brainwashing involving the use of Tracker Jacker venom that induces extreme fear. Peeta has been programmed to hate and fear Katniss. After he attempts to kill her, he is kept restrained under heavy guard at all times as the rebel medics seek a way to restore him to sanity.
A controversial strategy proposed by Gale wins a decisive victory at District 2, readying the rebels to launch a final campaign against the Capitol itself. Katniss and her propo team (a film crew recording her squad for rebel propaganda) are deployed on a trivial assignment to the Capitol. President Coin has sent Peeta along on the mission even though he is still dangerous and unpredictable. Katniss believes Coin wants her dead because she is difficult to control.
While filming in a purportedly safe Capitol neighborhood, the team's commander, Boggs, is killed. Subsequently, during intense urban warfare that involves Hunger Games-like monsters, most of Katniss' team mates, including Finnick, are killed. Katniss presses on alone towards President Snow's mansion. As Katniss reaches the mansion, a hoverplane drops parachutes carrying bombs that explode among a group of trapped children. The rebel medics (including Katniss' sister Prim) rush in to help the injured children but in a deliberately inhumane strategy, a second wave of firebombs explodes, taking out the medics. Prim is killed and Katniss sustains severe burns, but the rebels succeed in taking the Capitol.
During her recuperation, Katniss becomes deeply depressed over her sister's death and confronts prisoner Snow. Snow claims that Coin orchestrated the bombing that resulted in Prim's death, persuasively arguing that Coin made it appear to be Snow's work in order to turn his supporters against him. Katniss realizes in horror that the strategy used to kill the medics had been developed earlier by Gale. When Katniss confronts Gale about his possible involvement, he merely expresses uncertainty. Katniss becomes convinced that rather than establishing a republic governed by representatives from each of the districts, Coin intends to assume Snow's power and maintain the status quo.
On the day Snow is to be executed, President Coin asks the remaining Hunger Games victors to vote on punishing the Capitol just as the Capitol punished the Districts: by holding a final Hunger Games that will target the children of the Capitol's leaders. Katniss silently reflects that this is exactly the same evil the Capitol committed when the last rebellion failed. Then, she votes "yes—for Prim." Haymitch is the only one who understands her hidden meaning. They proceed to the execution, where Katniss is to shoot Snow. As she readies her bow, Snow flashes her one last smile, reminding her of his promise to always be truthful to her. Katniss silently agrees and, making her decision, she raises her bow and shoots Coin instead, killing her. Katniss immediately attempts suicide, but Peeta stops her and she is arrested during the ensuing riot. After the riot, Snow is found dead. Katniss is ultimately acquitted of Coin's murder by reason of insanity, and she is relocated to the ruins of her home in District 12.
Months later, Peeta and some other District 12 natives also return there. Peeta has gradually recovered the memories of his love for Katniss. Katniss embraces her love for Peeta, recognizing her need for his hope and strength. Together, they write a book in order to preserve the memory of those who died. Peeta still suffers flashbacks from his hijacking and Katniss still wakes up screaming from nightmares. By helping overthrow Snow and assassinating Coin, Katniss has preserved the possibility of Panem transitioning to a representative form of government, but at a tremendous personal cost.
In the epilogue set 20 years later, Katniss and Peeta have two children. The Hunger Games are over for good, and the children represent hope that future generations will benefit from the sacrifices of their parents. Katniss dreads the day her children learn about their parents' involvement in both the Games and the war. When she feels distressed, Katniss plays a comforting but repetitive game: reminding herself of every good thing she has ever seen someone do. The series ends with Katniss' somber reflection that "There are much worse games to play."