In the opening scene, Katniss Everdeen (Jennifer Lawrence) is getting vocal therapy after being nearly choked to death by Peeta Mellark (Josh Hutcherson) in the previous film. Once she can properly speak, she sees her younger sister Primrose "Prim" Everdeen (Willow Shields) entering Peeta's cell. The programming inflicted on Peeta in the Capitol eventually makes him freak out. He thinks that Katniss is a dangerous Capitol-created mutation, and tells Prim to kill Katniss.
Katniss goes to District Two with her military escort to film more propaganda videos for the rebel cause. Gale Hawthorne (Liam Hemsworth) accompanies her. He admits that he has seen Peeta, and realized that if he stays this way, Katniss will always chose him over Gale out of pity. On the way, she kisses Gale, but he calls it "the kiss of a drunk person." She jokes that with Peeta safely in Thirteen, he doesn't have to be jealous anymore.
Katniss overhears Gale and Beetee (Jeffrey Wright) discussing a dubious trick they mean to employ. It involves setting off a bomb that will kill many people and force the survivors to flee, and then deploying a second bomb in their direction to maximize the number of victims. Once in District Two, Commander Paylor (Patina Miller) mentions that the Rebellion failed to capture the arsenal from the Nut, a huge mountain containing Capitol infantry. Gale suggests that they use the special bombs that he and Beetee designed to blow up the Nut and take it over from the inside. Katniss protests, saying that there are a lot of civilians in there, but the rest are convinced that those people made their choice by collaborating with the enemy.
When the hovercrafts go, Katniss says that it's horrible as lives will be lost. Gale tells Katniss that this is war. Katniss is rushed off to shoot a propaganda speech, and then the captured refugees arrive by train. The rebel troops treat the refugees with suspicion, ordering them to drop their weapons and shooting some who don't comply. Katniss stops her speech and screams at everyone to stop.... and then a refugee holds her hostage. He tells Katniss, "Give me a reason why I shouldn't shoot." Katniss replies that she can't, but that the Capitol has been setting the Districts against each other, providing them with plenty of reasons to kill each other. She's so tired that dying would be a relief. The refugee lets her go and Katniss begins an anti-President Snow speech, only to be shot. Chaos ensues.
In the Capitol, President Snow (Donald Sutherland) celebrates with his delegates with a toast at his dinner table, but they all drink without stating what it is they're drinking to. Snow asks one of his ministers, Antonius (Robert Knepper), what they are celebrating. He replies that it's Katniss's death. Snow replies that if Katniss were really dead, the resistance would have made a martyr out of her already. Antonius then says that they toast since Katniss has failed. Snow disagrees and tells him that they are celebrating the ending of an era. Antonius drops dead, having been poisoned.
Katniss wakes up from her injuries to find a bald, morphine-crazed Johanna Mason (Jena Malone) in her room. Johanna is mad at Katniss and blames her for everything, although she admits her anger is partially jealousy, since Katniss was always intended to be irreplaceable for the uprising. She pulls out Katniss's morphine drip and attaches it to her own arm. Katniss manages to ease Johanna's sarcasm by admitting that Johanna would have made a better Mockingjay.
Next, Katniss visits Peeta, who's still locked in a cell and strapped to his bed. Peeta is no longer enraged but still blames Katniss for everything, going so far as to say if he'd never given her the overdone bread from his parents' bakery, they would never have ended up in that state.
Katniss is visibly upset and goes on to a conference with President Coin (Julianne Moore) and Plutarch Heavensbee (Philip Seymour Hoffman). Katniss wants to go to the Capitol and fight, but Coin, saying she's in no shape to fight and too valuable to lose, orders her to stay and shoot more propaganda films. Plutarch offers no objections, so Katniss agrees.
Katniss then attends the wedding of Finnick Odair (Sam Claflin) and Annie Cresta (Stef Dawson). At the reception, everyone is happy and dancing except Katniss. Johanna finds her brooding and when Katniss says she wants to kill Snow, Johanna offers to cover for her and suggests a plan: sneak on board a medical supply hovercraft headed for the Capitol. Katniss goes off to dance with her sister Prim, hugging her before following Johanna's plan.
When the hovercraft drops Katniss with the medical supplies outside the Capitol, Katniss tries to be unobtrusive but is recognized by almost everyone she encounters. She finds Gale and people crowd around, curious about why she's there. Commander Paylor makes a speech. The Capitol is surrounded by rebels and its citizens are abandoning the streets. The Peacekeepers have placed deadly "pods" all over the Capitol. The pods are traps armed with deadly weapons, similar to traps used in the games created by the Capitol. Being unable to control her, President Coin has integrated Katniss into Squad 451, whose job it is to shoot propaganda videos while locating and destroying the pods. They are ordered to remain behind the front lines to stay as safe as possible, while boosting morale with their propaganda videos. The squad is commanded by Boggs (Mahershala Ali) and includes Katniss, Gale, Finnick, filmmaker Cressida (Natalie Dormer), Homes (Omid Abtahi), Lt. Jackson (Michelle Forbes), Mitchell (Joe Chrest), Messalla (Evan Ross), the twins Leeg 1 and 2 (Misty Ormiston and Kim Ormiston) and Castor (Wes Chatham) and Pollux (Elden Henson), an avox (mute). They are each given a Nightlock pill, which causes instant death if they need to commit suicide to stay out of the Capitol's hands. Boggs has been issued a holo, a device that displays a holographic map and helps detect pods; however, the information on the pods is based on intelligence from spies, and may be outdated. The holo can also act as a bomb if the word "Nightlock" is repeated three times (important later).
The squad assembles and finds the first pod. Katniss fires an arrow toward it, which triggers a giant explosion. The squad decides to camp out in an abandoned restaurant for the night, when Peeta is unexpectedly dropped off by a military escort. Katniss is on edge, but the soldiers delivering him explain that President Coin wants Peeta in the videos as well, to show him actively fighting the Capitol alongside Katniss. Boggs takes Katniss aside and tells her about Coin. He says Coin wanted Peeta saved and not her; now that victory is at hand, Katniss may be a liability for Coin, who wants to be the next President of Panem. Though Katniss has no political aspirations, her allegiance to any candidate carries considerable weight. Because Coin can't be sure Katniss will endorse her, she prefers to be rid of Katniss now. She may have added Peeta to the team in the hope that he would kill her in a fit of rage and fear. Boggs tells Katniss to watch out.
Night falls and Peeta can't sleep. He and Katniss talk, and they play a game to help Peeta figure out what is real and what parts of his memories are planted by Capitol programming. As day comes, Finnick helps Peeta as they explore the city, entering an oval courtyard surrounded by abandoned apartment buildings. They're getting ready to shoot a propo. A pod of machine guns appears, but everyone dodges the hail of bullets. As they walk forward, Boggs is suddenly shot in his chest and Leeg 1 takes a bullet in the leg. Boggs, with his dying breath, gives Katniss full security access to the holo. He also tells her not to trust them, and to kill Peeta if necessary. Leeg 2 accidentally activates a trigger hidden under the pavement, releasing a torrent of black ooze which floods towards the team and quickly submerges the courtyard. While escaping, Peeta becomes frightened; his Capitol programming is activated and he tries to kill Katniss with a bow. Mitchell intervenes and in the process, Peeta shoves him in the ooze which causes him to fly up and get impaled by spikes.
The surviving members of Squad 451 run inside a building. Jackson, as Boggs' second in command, demands the holo from Katniss. Katniss says that Boggs transferred the voice-activated holo to her (which is true) and that she is on a personal mission to kill Snow, on orders of President Coin (which isn't). Cressida backs up Katniss' story, saying that Plutarch wants the assassination of Snow to be televised, hoping that it will quickly end the war. Jackson doesn't believe it and commands Katniss to transfer the holo to her (Jackson). Katniss shows her it's programmed for her, but Jackson still has doubts. A Mexican standoff occurs, but eventually Jackson backs down and allows Katniss to keep the holo as long as she obeys orders. Leeg 2 decides to stay behind with her injured twin; Jackson promises to send help as soon as they can. Peacekeeper soldiers arrive as soon as the rest of the squad exits the building and they take refuge in a vacant apartment across the courtyard. They watch in horror as the Peacekeepers blow up the building they just left, killing the Leeg twins.
Peeta feels extremely guilty over snapping and killing Mitchell and nearly killing Katniss. Suddenly, a broadcast pops up. Caesar Flickerman (Stanley Tucci) announces the death of Katniss and her squad as the building blowing up is televised. Snow pops out and says that he's happy to see Katniss dead and that the rebellion is over. Then Coin hijacks the broadcast, defending the Mockingjay (Katniss) and asserting that she is still the face of the revolution, even in death.
The squad heads for the president's mansion, but the holo shows multiple traps on the street. Pollux signs to Castor that they can go down into the sanitation system instead, a place where he did slave labor and is reluctant to go back to, but they have no choice. Underground, they hide from the peacekeepers but are seen on the security camera. President Snow, whose nose is bleeding, then sees the security camera footage and is surprised.
In the sewage system, after narrowly avoiding a steam pipe burst, the team decides to sleep. Katniss indulges Peeta in more games of 'Real or Not Real' and then Peeta reveals that the Capitol used wasp Tracker Jacker venom in his programming. Katniss promises to keep them alive. Katniss then hears something and the group investigates. Something has been sent after them.
After some intense searching (almost like a horror movie), suddenly dozens of hideous, lizard-human hybrid creatures which are called "mutts" (short for mutants?) appears from all directions and attack. Jackson is quickly overwhelmed and is either torn apart and/or bitten to death by the weird creatures. The remaining group fights them off and tries to escape. Castor is trampled and killed by the creatures. Cressida and Messalla are separated from the rest of the group as more creatures converge on them. The squad is corralled into a junction area space with a ladder leading up to the ceiling. Here, Katniss is nearly killed by one creature but she and Peeta fight them off (she with her bow and explosive-tipped arrows, and he with his assault rifle) as they stand back to back fighting. Cressida and Messalla reappear where they assist the group and order them to escape up the ladder. Cressida takes on and guns down several of the creatures herself, showing more of her fearless combat skills. Finnick uses his trident to fight off the creatures while the remaining members of the squad escape up the ladder. Unfortunately, this turns out to be his sacrifice; as he tries to ascend the ladder, the creatures overwhelm him. Not wanting Finnick to die in vain, Katniss says "Nightlock" three times and throws the holo into the melee of mutts and a screaming Finnick. The holo explodes, killing the creatures and ending Finnick's suffering.
In an underground space that looks like a subway station, the surviving members of the squad run across a floor, fired upon by Peacekeepers. As the floor begins to collapse, lethal beams in the light fixtures kill those who pass under them. Messalla runs into a beam and disintegrates into black ash while Peeta panics, thinking he's about to activate again. Katniss stops and kisses him, which fixes him right up. They get out to the surface and Cressida takes them to a fashion shop owned by Tigris (Eugenie Bondurant), who sports elaborate tiger-stripe tattoos and has apparently had some surgery to make her mouth and nose look more feline. She hides the five surviving members of the squad in a secret basement under her shop. Katniss eventually recognizes Tigris as one of the stylists who worked with contestants at the games. Tigris indicates that her tiger makeover was forced on her by President Snow "because I wasn't pretty enough any more."
While hiding underground, Pollux weeps over the loss of his brother Castor, while Cressida tends to Gale's wounds, sustained while fighting the mutts. A distraught Katniss says that her mission was fake and that she feels responsible for all those deaths. Cressida tells her that they all knew she wasn't on a mission ordered by Coin. But everyone, even Jackson, decided it was worth the risks. Peeta mentions his Capitol programming and tells Katniss to end all of it. He tells her that if Snow dies, all of the deaths will be for something. He tells Katniss to handcuff him again to be safe from him.(Peeta has been handcuffed most of the time he's been with Katniss's squad.)
While Katniss tries to sleep, Gale and Peeta reconcile. Gale tells Peeta that she loves Peeta, not him ("she never kissed me like that"). An emergency broadcast pops up with President Snow ordering Capitol residents to take shelter in the presidential mansion -- the Rebellion has arrived. As the pods near the mansion will presumably be deactivated, Katniss decides to walk with the refugees and take down Snow once and for all. Gale decides to go with her; Katniss uncuffs Peeta.
While Cressida and Pollux will have to stay behind since their rugged appearances are too noticeable -- and someone needs to watch Peeta -- Katniss and Gale, wearing coats with hoods that partially obscure their faces, march towards Snow's mansion with the other refugees. Katniss is almost recognized by a little girl who is carried by her mother. When they see Peacekeeper soldiers checking everyone's faces, they try to go back, but are forced to follow the crowd when the Peacekeepers start closing in from behind as well. Just as a Peacekeeper is about to expose Katniss, a nearby tank explodes -- the Rebellion soldiers have arrived in the Capitol. Katniss sees the little girl weeping over her dead mother. Gale is captured by the Peacekeepers and dragged away. He screams at her to shoot him with her bow, but she doesn't. Katniss runs to the front of Snow's mansion, where she sees parents passing their crying children from hand to hand through the crowd to get them through the mansion's gate -- children were the first priority in the evacuation. Then suddenly, a plane carrying the Capitol logo drops dozens of small, odd silvery pods on little parachutes. The crowd watches them fall; some people even reach out for them. The pods explode, killing many in the crowd, both adults and children. Katniss sees Prim, now a combat medic, trying to tend to the children. She screams a warning at Prim, only for another batch of bombs to drop and explode, killing Prim and knocking Katniss unconscious.
A wounded and burned Katniss wakes up inside Snow's mansion while her mother (Paula Malcomson) tends Katniss's wounds. Haymitch (Woody Harrelson) informs her that the rebellion was successful, and that Coin is now leading them. From the moment that the Capitol bombs fell on the citizens, the Peacekeepers immediately turned against the Capitol, and the palace was taken within a short time. From her mother's wordless tears, Katniss learns that Prim died.
When she's in shape again, Katniss explores the mansion and eventually goes to the garden, where Snow is held under guard in the conservatory. Though no one is allowed access, Commander Paylor grants her an audience with him. Snow is his cheery old self, as always, not seeming to mind his defeat. He tells Katniss that he was not responsible for the parachute bombs; he may not be above killing children, but he does not approve of waste either. He reveals that the bombs were part of Coin's secret plan to discredit him, and they worked -- even his guards turned against him for killing the children. He coughs up some more blood and wipes it away with his handkerchief. Katniss says in disgust, "I don't believe you." Snow answers, "Oh, my dear Miss Everdeen, I thought we had agreed not to lie to each other."
Katniss is awaiting an audience with Coin. She meets first with Gale. Katniss indirectly blames him for Prim's death, as the circumstances of her death closely resemble the 'bomb trap' she heard him discuss earlier. He admits that he honestly doesn't know whose bombs killed Prim, though he apologizes for not protecting her. Katniss bitterly says "Goodbye, Gale." A meeting takes place with Coin and the surviving victors. Coin has appointed herself interim President of Panem and says she'll call elections when the time is right, but the people are currently in no state to vote yet. Snow is to be executed, but Coin fears that this will not satisfy the need for vengeance among the citizens of Panem. She has an idea: she wants to have another Hunger Games, but with Capitol children. She asks the victors to vote on it. Peeta, Annie, and Beetee are strongly against the idea, but Johanna and two other victors are in favor. It's all up to Katniss and Haymitch. After demanding that she be allowed to execute Snow, which Coin agrees to, Katniss says 'yes' for Prim, although she becomes aware of Coin's motive. Haymitch, after looking at Katniss as though she's lost her mind, nonetheless says "I'm with the Mockingjay," meaning the vote goes in Coin's favor, 4 to 3. Effie Trinket (Elizabeth Banks) dresses up Katniss for the execution, and Katniss pockets a Nightlock pill.
Katniss marches alone into the arena, the large parade ground where the tributes used to be showcased, and where Snow is bound to a pole. President Coin (who is conveniently standing on a stage just beyond Snow) makes a speech before saying to Katniss, "May your aim be as true as your heart is pure." Katniss aims her arrow at Snow, who appears unfazed. Thinking of her last conversation with Snow and the meeting with Coin before the execution, Katniss realizes that Snow was telling the truth about Coin: she ordered the bombing of Capitol refugees that caused Prim's death. So rather than shooting Snow, Katniss turns her arrow and shoots Coin, killing her. The crowd is shocked as Coin tumbles to the ground, dead, while President Snow almost chokes from laughter. As the livid crowd charges towards the laughing Snow, Katniss tries to commit suicide by taking the pill, only for Peeta to smack it out of her hand. Katniss is dragged off and taken into custody as the crowd reaches Snow and they brutally beat him to death.
Katniss is placed in a vacant, guarded room where Haymitch soon arrives and reads a letter written to her by Plutarch. Plutarch says that he regrets not meeting face to face, but in the light of what happened, it is better if he is not seen with her. He expresses his approval of what Katniss did. Nevertheless, she will be exiled for some time to her old District Twelve. He says the leaders of the districts are choosing an interim leader to replace Coin. It will probably be Commander Paylor, who will eventually pardon her for killing Coin. Plutarch says at the end: "We may mess up, but perhaps we may learn from our mistakes this time."
Katniss is sent back by train to her home in the Victors Village in District Twelve, accompanied by Haymitch. After saying goodbye to Effie, Haymitch tells Effie not to be a stranger and kisses her.
Back in Twelve, when Katniss enters her home, she sees Prim's cat Buttercup in the kitchen. She has an emotional meltdown over Prim, shouting at the cat that Prim is gone and never coming back. Then she hugs the cat and cries. She slowly builds herself up again by hunting. One day she finds Peeta planting primroses in her garden -- the flower her sister was named for. They hug and then after a montage of them living together, Peeta asks Katniss: "You love me. Real or not real?" Katniss answers: "Real."
Several years later, Katniss and Peeta are married and have two children. Peeta is playing with the older child while Katniss peacefully holds the sleeping baby in the same field that opened the first Hunger Games film. The baby wakes up and fusses a little and Katniss asks, "did you have a nightmare?" She says someday she'll tell her children her story, and that nightmares are like a game played over and over again, and can feel tedious at times... "but there are worse games to play."