This review will include spoilers to The Hunger Games, in other words, stop reading if you don't want to know how it ends.
The sequel begins about six months after the conclusion of The Hunger Games, and only a few hours before the Victors Tour that takes place halfway between the annual Hunger Games (have to keep the audiences happy and interested, after all). Katniss is still adjusting to the changes in her life and her relationships. She and her family now live in the Victor's Circle with only Peeta and Haymitch as neighbors, the only other living District 12 winners. Before the tour kicks off, President Snow, leader of the Capitol, makes a personal visit to Katniss and warns her she needs to watch her behavior on tour, threatening the lives of her friends and family. At the conclusion of last year's Hunger Games, Peeta and Katniss had partnered up upon the announcement that for the first time ever there could be two victors if both tributes from the same district were left standing. As soon as Peeta and Katniss were the last survivors, this revision was revoked, and rather than kill Peeta or let him kill himself to save her, she made the gamble that the Games had to have a victor, and asked him to eat poisonous berries with her. Rather than have no victor, the head game maker reneged the previous statement, and crowned both Peeta and Katniss. While most of the audience in the Capitol have read this gesture as the romantic gesture of a young girl madly in love, some of the districts have interpreted it as an act of defiance against the Capitol, and have drawn inspiration from it. President Snow wants Katniss to present herself as a young girl in love to convince the districts that she in fact did not defy the Capitol, and quell the growing signs of unrest and discontent.
Katniss herself isn't completely sure what her intentions were with her action, whether it was out of affection for Peeta (definitely not mad love), in defiance of the Capitol or cold calculation on her part - District 12 never would have forgiven her or let her live in peace if she had killed such a decent human being they all knew - the only thing that is clear to her is that she thought it would allow her to survive, and she wasn't doing it because she wanted to sacrifice herself. The first visit on the tour is to District 11, which her friend and ally Rue had represented. Katniss sees the anger in the crowd, and understands what President Snow was referring to - Peeta's and her speeches lead to a show of solidarity, and she witnesses several civilians killed in response. Following that incident, Peeta and Katniss play along as much as possible with President Snow's desires to prevent more deaths, but at the end of the tour, Katniss is told that she has failed to calm the feelings of unrest and rebellion.
After returning home, Katniss worries about the repercussions for her family, but also starts to hear more vague rumblings about uprisings. Even though the media is completely controlled by the Capitol, she gathers that District 8 has rebelled based on a report she accidentally sees in the District 12 mayor's house. She also runs into two refugees from District 8, and begins to hear rumors that District 13 may not have been destroyed after all. In addition to all these vague rumors, it also very clear that the Capitol is cracking down since the security force has new leadership and an increased presence in District 12.
Finally, the 75th Hunger Games are steadily approaching, and every 25th anniversary is referred to as the Quarter Quell, bringing even more horrors with it. For the 25th anniversary, the districts had to vote on which children would serve as tributes rather than the usual draw, for the second Quarter Quell, the Capitol drew twice the number of tributes (this is the year Haymitch won). Between the anxiety and anger over the Quarter Quell and the general oppression, the country is a powder keg ready to explode.
While The Hunger Games dealt with personal survival within impossible circumstances, in Catching Fire, Katniss witnesses the beginnings of a revolution. She vaguely begins to understand that she has become a symbol of defiance, and a symbol for a movement since she defied the Capitol and survived. As the novel progresses, several new characters are introduced, especially after the rules for the Quarter Quell are announced, and the drawing afterwards (I quite enjoyed Finnick). Instead of simply worrying about her own survival, Katniss has to worry about her family's surival (and not just keeping them fed as before) and has to learn where the line is between survival at all costs and standing up for a cause. She blames herself for many of the deaths she sees (such as in District 11), even if they were due to people's choices and the Capitol's regime. From reviews, I had heard that this novel wasn't quite as good as the first, and while it wasn't necessarily as quickly paced or action-packed, I actually quite enjoyed reading about the politics, and learning more about the districts. I felt like there was more than enough going on, and in fact, more happened than in the last novel, it just simply wasn't all action. Collins did a great job of presenting how in a disillusioned and oppressed society, one simple gesture or act can be all that it takes to return hope and give people something to fight for - obviously, once logistics are involved, it becomes a lot more complicated but what was once an impossibility can become a choice with the right spark.