This synopsis will contain spoilers!
While still recovering from her injuries at the end of the Quarter Quell, Katniss has returned to the remnants of District 12. She sees the destroyed town, but learns that her victor home is still intact. She gets her father's hunting jacket and rescues her sister's cap Buttercup. Back in District 13, Katniss is struggling with the rigidity of militarized life in the district while being pressured to become the Mockingjay - the symbol of the revolution against the Capitol. She finally agrees to, but only under the condition that Peeta and the other captured victors are given immunity upon their rescue. Coin, the President of 13, reluctantly agrees.
In order to display her as the Mockingjay, 13 begins recording propos (propaganda videos) of her in action. The first will be a visit to District 8 after a bombing where Katniss interacts with the patients in a hospital. It is immediately evident that her presence is uplifting to all the rebels. While they are there, the Capitol bombers return, and Gale and Katniss are able to shoot several out of the sky (thanks to modified bows created by Beetee. The hospital is destroyed.
Katniss' role as the Mockingjay solidifies the rebel army, and they begin to take all the districts (except 2). Meanwhile, Katniss is troubled by videos of Peeta in which he calls for a cease fire. In them it is clear he is being tortured. Finally, after warning the rebels of a bombing in 13, Peeta is beaten mercilessly on camera. Katniss begins to go mad. After the bombing in 13 is over with, Coin and the others decide they must rescue Peeta to put Katniss at ease. They succeed, but Peeta has been hijacked (brainwashed) and attempts to kill Katniss upon seeing her.
District 2 is the only District still under Capitol control thanks to a mountain stronghold they have. Gale has the idea of causing an avalanche, trapping everyone inside. It succeeds, but a small group escapes. When Katniss tries to help them, she is shot. When she recovers, she decides to train to be a real soldier, because it is the only way she will be able to be a part of the invasion of the Capitol. She is put on a sharpshooter squad with Gale and Finnick under Boggs' command.
In the Capitol, they must move slowly to destroy booby traps (pods), but her squad is not really allowed into combat due to their celebrity status. One day, however, Peeta arrives. Katniss and the others believe this is a sign that Coin wants Peeta to kill Katniss now that she is no longer of use alive. The Mockingjay martyred will do more to help the rebellion at this point.
While shooting a propo, everything goes wrong, and Boggs is killed. A black smoke rolls in to cover the street, and the squad enters an apartment. Peeta nearly kills Katniss. Boggs puts Katniss in control, knowing she wants to find and kill President Snow. She leads them back out through the smoke (which left a suffocating gel on the ground) just in time to avoid a bombing of the building they were in. The Capitol releases a video claiming the group is dead.
They continue on, and everyone but Peeta, Gale, Katniss, and part of the camera crew are killed. They escape to a spy in the Capitol who helps hide, and then dress them as refugees. They flee to the President's mansion, but Gale, Peeta, and Katniss are separated. Katniss arrives at the mansion, just in time to see bombs dropped on the children surrounding it. Her sister, Prim, arrives with the Rebel medics, only to be killed in a second wave of the bombs exploding. Katniss blacks out after being severely burned while trying to rescue her sister.
The rebels win, and Katniss is promised she can execute Snow. She comes across his prison cell, and he tells her it was not him who bombed the children and killed her sister, but Coin. Katniss is not sure who to believe. Before the execution takes place, Coin gathers all the surviving victors to take a vote on if they should hold one final Hunger Games with the children of the Capitol. Katniss and Haymitch say they should, but Peeta says no. The yes votes win. At the execution, Katniss realizes it was Coin responsible for the bombs, and so she shoots her instead of Snow.
Katniss is tried, but found mentally unstable and so not killed. The general from District 8 is elected President. Katniss returns to 12, with Peeta and Haymitch. Gale does not, because the bomb that killed Prim was designed (though not deployed) by him and Beetee. He realizes that Katniss will always connect him to Prim's death as a result
In the epilogue, it is many years later, Katniss and Peeta have 2 children (a girl and a boy), and there has not been a Hunger Games since the Quarter Quell.
Just as I did with the first 2 books in the series, I enjoyed Mockingjay and finished it in just a few days. It was a fast, easy read that …- Sept. 30, 2010