The movie picks up right where Part 1 left off. Katniss was attacked by Peeta after he was brainwashed and tortured by President Snow and the Capitol. They're trying to fix him but it isn't an easy process and he still lashes out and hates Katniss. She vows that she's going to kill Snow for what he did but before she can even think about it, District 13 needs to be able to access The Capitol. To do that they need to take over a weapons armory in District 2, which is fortified and basically a huge fortress under a mountain.
Gale, President Coin and Beetee basically come up with this elaborate bomb plan to be used later in the war effort. For the more immediate problem, Gale suggests causing an avalanche to block the fortress (since they can't get inside, it's too heavily fortified) and cut them off. Katniss protests because there's civilians that would become trapped inside the place so Coin compromises and decides to leave a train entrance untouched so that people who want to leave can do so. It pretty much goes without a hitch. A train arrives, people get off, one attacks Katniss, a different one actually shoots her (but her armor is bullet proof, naturally). But in the end, District 13 captures District 2 and that basically means all of the districts are now reunited and stand behind 13, poised to attack and take over the Capitol. Katniss' job is finished.
But it's not, because she's the main star. She sneaks aboard a medical transport and wants to head off into the Capitol alone on an assassination mission. She's seen once she arrives back in District 2 though, naturally, because everyone knows who she is. Now she's not able to go alone and President Coin decides that she'll make that assassination attempt seem like it came from her. They assign her to a military squad, one that is supposed to bring up the rear and is for propaganda ("propos") point only. And from here is where I start to have an issue with the film. Up to this point it was fine.
Everything that happens from this point to the very end of the rebellion's attack on the capitol is more or less pointless. Everything would have ended exactly the same if Katniss had just stayed in bed back at 13 after she had been shot. In fact, it probably would've turned out better.
See, the rebels go on ahead (a day's worth ahead, I think they said) and the propo team follows behind, mostly empty streets, occasionally having to dodge a trap or two that has been set to defend the city. Katniss still wants to break off on her own but they need a machine that tells them where the traps are (which has security so that only one person can use it). Eventually Peeta is assigned to the team and while he's better he still has occasionally moments where he snaps. Unfortunately for the group, they trigger an undetected trap and a few people from the group die. They are chased into a building by this oil-looking substance that instantly kills you if it catches you. It also alerts the Peacekeepers as to where they are. When the oil subsides the members of teh group that are able decide to flee into a different house to hide and the Peacekeepers show up and blow the crap out of the first building. This leads the Capitol, and Snow, to believe that Katniss is dead, and they do a "Tribute" to them on the TV (just like they would in The Hunger Games where they'd show the picture with the name and all that).
With everyone believing they're dead, Katniss and gang decide to head underground to try and move unseen. Unfortunately a security camera catches them and Snow unleashes the hounds--hounds in this case being weird alien-looking zombie people that swarm the group causing a few others to die. The group escapes the sewers where the Peacekeepers are waiting for them and they lose a few more members of the group. The film maker (Cressida, I think her name is) leads them to a woman who gives them safe harbor for a night. Meanwhile the Rebels have breached most of the city so Snow orders an immediate evacuation to his mansion. He claims he'll protect them and give them shelter/food/clothing. In reality, I'm pretty sure he was planning to use them as human shields. Regardless, Katniss and Gale decide to play some Assassin's Creed and get these huge hooded coats and blend into the refugees. Unfortunately they forgot that peacekeepers would be there and would be removing people's hoods.
When it looks like the gig will be up and they'll be discovered, the rebels arrive and start blowing crap up. Katniss gets up and watches as the peacekeepers start taking children to try to get them into the mansion. Then she watches as a plane flies by and drops a bunch of can things on parachutes. As soon as the parachutes touch the refugees they explode, causing a massive carpet bombing of everyone near the gates. Katniss, who is dazed and nearly knocked out at this point, manages to see her sister on the front lines of the medical team trying to aid any of the survivors near the gate. As Katniss goes to approach her, a second bomb goes off killing everyone that is anywhere near the gate. Katniss blacks out, having nearly been blown to smithereens.
Upon recovering, Katniss learns that the Capitol has been conquered by the rebels, and that Snow has been captured. Katniss confronts Snow, who claims that Coin masterminded the bombings in order to turn his supporters against him. When Katniss accuses him of lying, he reminds her of their promise to always be truthful to each other. Suspicious, Katniss realizes that the final bombs that ended up killing her sister resembled the bombs that Coin, Gale and Beetee were working on towards the beginning of the film. Gale confesses that while he's not 100% sure if it was that particular bomb, he says he's failed to do the one thing Katniss asked of him: protect her family. Angry and disappointed, Katniss sends Gale away.
President Coin (who has appointed herself interim President of Panem) and the remaining Victors hold a meeting to discuss having one final edition of the Hunger Games with the children of the Capitol as retribution for the previous games. Realizing that Snow was right and that Coin has "played them both for fools", Katniss votes in favor of the motion "for Prim," which gains Coin's trust. She is awarded the opportunity to execute Snow.
At the execution, which is being held before the whole of Panem, Katniss faces Snow once again. As she readies her bow, the two make eye contact, and Snow gives her one last smile. Katniss silently agrees, and instead shoots the arrow into Coin's heart, killing her. A lynch mob bumrushes and kills Snow while Katniss is taken into custody.
Katniss is pardoned for her crime, being deemed mentally unwell, and through a letter delivered by Haymitch, Plutarch Heavensbee advises Katniss to return home to District 12 until things settle down. Katniss returns to her home in the ruins of District 12 to recover from her traumatic ordeal, where she is eventually joined by Peeta, who has recovered his memories of love for Katniss. The people elect a new President of Panem, and Katniss, Peeta, and Haymitch bond over their shared trauma. Katniss and Peeta slowly grow back together, and Katniss admits her love for him. Years later, Katniss and Peeta are shown to have two children. Katniss reminisces about her recurring nightmares she still suffers from and explains she plays "a game" where she lists all the good things she has seen someone do. She notes that while the game has grown tedious over the years, "there are much worse games to play", as she smiles into the camera before the screen cuts to black.