This synopsis will contain spoilers!
Peeta and Katniss have not spoken much since the end of The Hunger Games, but they must continue the charade of being in love soon with the Victory Tour beginning again. Katniss has only see Gale on Sunday in the woods because he works in the mines during the week. They pretend to be cousins to the Capitol and the other districts.
Just before the Victory Tour starts, Katniss is visited by President Snow, who tells her he knows everything, and that if she wants her family and Gale to stay alive, she must convince even him that she loves Peeta. This is the only thing, Snow claims, that will stop all the districts from rising up. On the tour, it is clear that many of the districts are about to revolt, and there was nothing Katniss could have done to stop it.
This year will be the 75th Hunger Games (a quarter quell), and so it will be special. It is announced that this year's tributes will be selected from past winners. This means Katniss will be selected, along with either Peeta or Haymitch. Upset, Katniss racing into the woods to go to the lake, where she finds two women who have escaped from District 8, which is in revolt now. They are headed to District 13 which, according to them, is still full of people living underground and fighting against the Capitol. Katniss is not sure if she should believe this, but she maintains some hope it is possible.
At the reaping Haymitch is selected, but Peeta immediately volunteers to take his place. Since she was protected by Peeta the first time through, she vows to protect him this time, with Haymitch's help.
Peeta and Katniss meet the other tributes during training, and Haymitch encourages them to form allies before the games begin. Katniss likes the old woman from District 4, and Beetee and Wiress from 8, but in the end she decides she doesn't want any allies. She and Peeta receive a ranking of 12, which is the Gamemakers way of making them the target of all the other tributes.
Just before the games begin, Katniss is forced to watch Cinna being beaten for the dress he made for Katniss that turned her into a Mockingjay during the interview. In the arena, the Cornucopia is surrounded by water. Katniss makes it to the stash of weapons and grabs a bow and arrow. Finnick, from 4, is there but he has Haymitch's bracelet so she knows he can be trusted. They form an alliance, along with Mags, the old woman from 4. Finnick must go get Peeta, however, because he can't swim. They flee into the woods.
They find food, and discover they can get water from the trees. A paralytic fog shows up and they flee to the beach. Mags dies. They run into Johanna who has brought Beetee and Wiress. With Wiress help they learn the arena is a clock, with different dangers appearing at regular intervals. They are attacked and Wiress is killed. Beetee reveals a trap to kill the others when lightning strikes a tree.
While executing the plan, Katniss and Johanna are attacked. Johanna protects Katniss and then cuts her arm (we later learn this is to remove the tracking device). Katniss makes it back to Peeta and Beetee at the lightning tree but he is gone. Beetee is unconscious. Katniss realizes the real plan was to destroy the force field around the arena when she sees the knife wrapped around the wire in Beetee's hands. She takes the wire and shoots it through the weak spot in the field (which was pointed out by Beetee back in the training area). The force field explodes, and Katniss and Beetee are taken up into a hovercraft.
Katniss awakes and sees Beetee next to her, still unconscious. She goes in search of Peeta, but finds Finnick, Haymitch, and the head Gamemaker, who was actually supporting the resistance. They reveal District 13 is, in fact, still around. There was a plan all along to break them out of the games because Katniss is so important to the resistance movement. Peeta, Johanna, and another tribute have been captured by the Capitol. Gale shows up a few days later to reveal that her family is safe, but District 12 has been destroyed.
I am so glad that Catching Fire did not devolve into a mere romance novel after the ending of The Hunger Games. The love triangle is still there, of course …- Jan. 15, 2010
Suzanne Collins the Last Line of in Catching Fire