"'Plutarch Heavensbee has been, for several years, part of an undercover group aiming to overthrow the Capitol..."
-Haymitch talking about Plutarch in Catching Fire
Plutarch Heavensbee was the a Gamemaker for at least one year in 74 ADD and was a Head Gamemaker in 75 ADD, and the last one as well. He was in an undercover group that that was planning to overthrow the Capitol for several years before the Hunger Games series would start to take place in, as mentioned above. He also became a commander of the rebel forces of District 13 sometime during Catching Fire.
The Hunger Games
He was briefly mentioned when Katniss shot an apple near Seneca Crane's head during her private session before The 74th Hunger Games would start. He was taken surprise when she shot the apple, and he fell backwards in a bowl of punch. When Katniss told Haymitch what happened during training, he laughed when she mentioned Plutarch fall in the bowl of punch.
Plutarch is formally introduced in Catching Fire, during a feast after the 74th Victory Tour. Katniss recognizes him as the man who fell in the punch during her private session. He attempted to win Katniss's trust, but she dismissed his discreet hints of rebellion, when showing the mockingly symbol. Katniss saw it and ponders what he meant by the sign, only to know what it meant later. Later he shows up as Head Gamemaker and keeps his cover low so he won't get in any kind of trouble.
Plutarch shows no thought of Katniss in public and treats her like all other tributes. He is surprised when both Peeta and Katniss go in their private training session when they show signs of rebellion, when Peeta painted a picture of Rue, and Katniss hung a dummy of Seneca. It comes as a surprise to Katniss when Plutarch is revealed as one of the leaders of the rebellion. Plutarch comes to the arena in a hovercraft and saves her, Finnick, and Beetee after a plan of his making removed the Capitol forcefield. However, they failed to save Peeta, Johanna, and Enobaria, but they would get saved later.
Plutarch is a leader of the rebellion and appears during the meeting held by Katniss when she accepts the role of being the Mockingjay. Plutarch is happy about her acceptance and that she hasn't changed, or at least yet. Plutarch then takes both her and Gale downstairs, but they are stopped on their way by a guard. Gale makes a distraction which allows them to see the guarded room and finds Katniss' prep team being locked up and tortured. Plutarch is surprised by this and demands them being freed.
Later, he goes along with the others to District 8 to shoot their first propaganda. During their visit to one of the hospitals, the Capitol attacks them. Plutarch tries to keep Katniss safe by not allowing her to fight, so she can just run. However, she does not accept this and goes to fight. After the propaganda is finished, Plutarch goes to receive medical treatment. They meet in the command room and discuss the propaganda. After that, they finish they decide to allow Katniss to go into combat again and film more after she has fully recovered.
Plutarch goes to Katniss's hospital room after Peeta's interview draws to an end because he's worried she is going crazy as a result. When he enters, Katniss and Finnick pretend not to have seen it. After Peeta's warning of the bombing on District 13 by the Capitol, Plutarch helps evacuate the entire district to safe bunkers so no-one will get killed in the bombing. Plutarch is happy during the bombing because Beetee was successful in hacking the Capitol and airing Katniss' propaganda.
Katniss does everything she can not to slap him. He informs Katniss and Prim of Peeta's condition and that he is receiving treatment for the injuries the Capitol has inflicted on him. Later, Katniss asks to go to the Capitol but he refuses since they need all the districts to be on their side first, but District 2 is still with the Capitol. Katniss tells him to send her to District 2 so she can persuade them to be in the rebel's side.
Plutarch shows up later after she is shot in District 2 and is happy to finally have all the districts on their side. Plutarch explains further about the relationship between the Capitol and the districts. He announces that the next propaganda they will shoot will be of Finnick and Annie's wedding. Plutarch gives the order of shooting new propaganda in the Capitol. Plutarch also tells her to get ready for training so they can record it.
Plutarch was also believed by Snow to be part of the plan that killed Capitol children and rebel casualty aides including Prim. However, this can't be confirmed. Katniss killed President Coinfor her part in the plan, but seeks no vengeance against Plutarch. He does Katniss a favor by preventing the execution of Effie Trinket and acting as a witness in Katniss's defense at her trial for the murder of Coin. Plutarch was later chosen as the Secretary of Communications after the war.