Klingon Chancellor Gorkon was murdered after seeking peace with the Federation, and Kirk and McCoy were wrongly convicted before exposing a conspiracy at Khitomer.
After the explosion of Praxis in 2293 crippled the Klingon economy, Chancellor Gorkon proposed ending decades of hostility with the Federation. The Enterprise-A was sent to escort his ship, Kronos One, to talks on Earth. After a tense dinner, Kronos One was struck by two torpedoes that seemed to come from the Enterprise, and two assassins in Starfleet uniforms beamed aboard and shot Gorkon, who died despite McCoy's efforts.
Kirk and McCoy were tried on Qo'noS, with General Chang as prosecutor, and sentenced to life on the prison asteroid Rura Penthe. Spock investigated and found that Lieutenant Valeris was part of a conspiracy of Starfleet, Klingon and Romulan officers, including Chang, who feared what peace would bring. Kirk and McCoy escaped and were rescued.
The conspirators planned to kill the Federation President at the peace conference at Khitomer. Chang's prototype Bird-of-Prey could fire while cloaked, but the Enterprise and Sulu's Excelsior destroyed it. Kirk stopped the assassination, and Gorkon's daughter Azetbur continued her father's work, leading to the Khitomer Accords.