Katniss is horrified by the way Peeta is being treated. The Capitol bombs District 13, just as Peeta warned it would. Citizens of District 13 retreat underground for several days. When it's all clear, Katniss makes another propo, while rebel teams go to rescue Peeta.
The rebels get Peeta and some other folks out of the Capitol, but Peeta's been badly damaged. He's been "hijacked" (as in, brainwashed) and now thinks of Katniss as the enemy. As soon as he sees her, he tries to kill her. Not-so-happy reunion indeed.
Once she recovers from Peeta's attack, Katniss needs to be part of the action. She heads back to District 2 and is part of a summit about whether to attack the other side humanely or inhumanely. Gale suggests some inhumane kinds of weapons and strategies. During the battle that follows, Katniss gets shot. While recuperating, she bonds with another victor who we met in Catching Fire, Johanna, and witnesses the wedding of victors Finnick and Annie.
After a second recovery, Katniss trains to be part of the fighting again. She, Gale, and Finnick are assigned to a special unit that will be filmed during combat and turned into more propos. Right before they go out into the field, Peeta's assigned to their unit too. Hmm. Is it just us, or does that seem like a bad idea?