The setting for all the novels is the future North America that has become the nation of Panem, comprised of twelve districts brutally ruled by a central capitol city ("The Capitol"). In return for their bare survival the districts work to provide the material needs of the Capitol. Some skirt the law and provide for themselves by hunting and black market trading. Among these are Katniss Everdeen, a seventeen year old girl struggling to take care of her mother and little sister after the death of her father. In The Hunger Games, we saw her participate in the annual fight-to-the-death of young people (called "tributes," aged 12 to 18) drawn from the twelve districts, when she volunteered to replace her young sister. This fight, (a game to the deluded and hedonistic citizens of Capitol) is done every year as punishment for the districts' rebellion many years before.
Katniss and her fellow tribute from District Twelve, Peeta Mellark, win the Hunger Games by a combination of bravery, smarts, and a ploy of invoking sympathy from Capitol viewers (it's all televised) by pretending they are lovers. The degree of that pretense, of course, becomes blurred. Still, it works, and it allows them to be co-victors instead of the traditional single winner.
The second novel, Catching Fire finds Katniss and Peeta living in the Victor's Village back home in District Twelve where they are provided a nice house, money, and plenty of food. This granted affluence, however, requires them to make a victory tour of the districts, with the added pressure of continuing their "star-crossed lovers" act so as to convince the populace, and President Snow, that their winning ploy in the Hunger Games was done out of their desperate love and not defiance of the Capitol.
Their act doesn't go so well when they are not able to keep up an adequate pretense of support for Panem in the face of the suffering they see in their tour of the districts. This leads to President Snow deciding to eliminate them by picking the tributes for the next Hunger Games from the pool of previous year winners. So Katniss and Peeta have to fight for the second year in a row. Since they are up against previous winners, their odds for survival are even worse than last year, and they won't be allowed to share a victory this time.
Catching Fire continues the socialist theme begun in the first novel which makes it a stand-out in the popular fiction being produced today. It is still written at a Young Adult level but is still complex enough to make it compelling to adult readers. In my opinion, Ms Collins has done a much better job with her prose in this book, which was a little too juvenile at points in the first. I also have to give her points for being a savvy storyteller. It would have been easy for her to make succeeding books adventures surrounding the next year's Hunger Games (as in each Harry Potter book being another year at Hogwarts) but she continues her story instead and it makes for a single, strong piece of fiction.
Without giving anything away (there may be somebody out there that hasn't seen the movie), Catching Fire starts like another Hunger Games but with the twist of a competition among the previous winners. The ending, however, is forced by the rebellion against the Capitol and the brave smarts of Katniss that cements her role as the rebellion's symbol. The book ends with an image of the next level of open rebellion, and an obvious pause in the storytelling. It begs not just for a sequel, but for a continuation of the story.
Catching Fire is that continuation, further developing characters, relationships, and situations in a satisfying and compelling way. It is the second part of an important story destined to become a classic in dystopian, populist literature.