The Khitomer Conspiracy was the name given to a group of personnel from the United Federation of Planets, the Klingon Empire, and the Romulan Star Empire who sought to stop the peace talks between the Federation and the Klingon Empire in the aftermath of the destruction of Praxis.
When the Klingon moon of Praxis exploded due to one of its quantum reactors overloading, the Klingons lost their key energy facility and their homeworld's ecosphere became imperiled. This left their empire with only fifty years of life left. When Gorkon challenged and succeeded B'rak as Chancellor of the High Council, he decided to open negotiations with the Federation.
After Trill Ambassador Curzon Dax and Kang gave Gorkon a personal receiver frequency, Vulcan Ambassador Sarek then had his son, Capt. Spock to covertly open dialogue with Gorkon. In the weeks following the disaster Gorkon recognized the Empire's precarious state, and through Spock, he proposed immediate peace negotiations with the Federation. Officially, the Federation and Klingon Empire embraced Gorkon's peace initiative, and Gorkon departed for Earth aboard the battlecruiser Kronos One.
However, some Klingon, Romulan, and even Starfleet factions were opposed to this. The Romulan Ambassador, Nanclus was given the order to undermine the negotiations and the proposed peace treaty. Using a romulan agent posing as a Starfleet officer, Laura Mogel, Nanclus and Adm. Lance Cartwright both agreed that the Neutral zone must be kept. The prospect of an end to nearly seventy years of hostilities, coupled with massive demilitarization, was not welcomed by some of the more conservative factions on either side of the Neutral Zone. Cartwright protested welcoming Klingon "trash", and insisted the proper course was to bring them to their knees and dictate a resolution on Starfleet's terms. Gorkon's chief of staff, General Chang, seemed to believe an outright war was more palatable than resolving the crisis politically. Both men would find support within their fleets, as well as from without, and joined in a conspiracy to subvert the peace process. However, Chang, despite his hatred of Gorkon, could not kill him without triggering a civil war in the empire. Thus Adm. Cartwright agreed to have Gorkon killed by his people. (TOS movie, novelization & comic adaptation: Star Trek VI: The Undiscovered Country; ST reference: Federation: The First 150 Years)
Following Starfleet C-in-C, William Smillie's meeting and assigning Capt. James T. Kirk and the USS Enterprise-A as the Federation's first olive branch toward the Klingon empire, Cartwright assigned three officers who he could trust to the Enterprise to carry out his and the fellow conspirators' plans. The Enterprise escorted Gorkon and Kronos One from Klingon space on the journey to Earth. A prototype Klingon Bird-of-Prey, capable of firing her torpedoes while cloaked, shadowed the vessels and attacked Kronos One, while an accomplice aboard the Enterprise forged the databanks pertaining to torpedo magazines (presumably Valeris), suggesting the Enterprise had fired. Lieutenant Valeris ordered Yeomen Burke and Samno to board Kronos One in the chaos, where they assassinated Gorkon. Valeris also covertly provided Chang with dialogue from Capt. Kirk's personal log to be used as evidence against Kirk. Valeris had calculated that Kirk would use this opportunity to attack and destroy the Klingon ship (PROSE: Cast No Shadow), but when Kirk and Leonard McCoy boarded the battle cruiser to offer aid, they were accused of murdering the Chancellor after McCoy was unable to save him due to inadequate knowledge of Klingon anatomy, arrested, and taken into Klingon territory for trial.
On Earth, fellow conspirator Colonel Patrick West, presented the Federation President with Operation Retrieve; an aggressive plan to invade Klingon space to rescue Kirk and McCoy before their trial, using 8 starships. Cartwright and Nanclus urged approval for the drastic action, but the President was wary that "Retrieve" might trigger full-scale war and refused it. In spite of the conspiracy's efforts, the peace process continued between the President and Gorkon's daughter, Azetbur, and a summit was scheduled to take place at a secret location.
Kirk and McCoy faced their show-trial, prosecuted by Chang himself. Though earnestly represented by Colonel Worf, the officers were convicted and sentenced to life imprisonment on the dilithium mines of Rura Penthe. Kirk found an "ally" in the comely Chameloid, Martia, who led the new prisoners to the surface and into a trap. Just before their attempted murder by the Prison commandant, Koth (another conspirator). Kirk and McCoy were rescued by Spock and the Enterprise-A.
Spock's shipboard investigation turned up the bodies of Burke and Samno, and exposed the treason of Valeris. Spock's forced mind meld with his erstwhile protégé identified the leading conspirators, and their new plan to assassinate the Federation President. From Captain Hikaru Sulu, they learned the location of the secret conference at Khitomer, on the Klingon border. (TOS movie, novelization & comic adaptation: Star Trek VI: The Undiscovered Country; ST reference: Federation: The First 150 Years)
The Enterprise, followed by the Excelsior, arrived at Khitomer, but then came under attack by Change's prototype bird-of-prey that fired under cloak. Chang kept them from coming within transporter range of Khitomer. Chang continued to fire at both the Enterprise and the Excelsior. However, Capt. Spock and Dr. McCoy were to modify a torpedo to track the Bird-of-Prey's ionized gas trail. When Kirk fired the torpedo, Chang's ship was hit and was destroyed by the Excelsior and the Enterprise.
Kirk and his staff then beamed down to the complex on Khitomer. At the conference, Cartwright and Nanclus had Adm. West surgically altered to appear as a disgruntled Klingon to assassinate Pres. Ra-ghoratreii. However, Kirk was able to save the President. Cartwright then tried to have Kirk arrested, but Spock told Cartwright to arrest himself, along with their witness Lt. Valeris. West then tried to assassinate Valeris. However, West was stopped by Kirk's Chief engineer, Montgomery Scott and fell to his death where he was exposed as human by Col. Worf.(TOS movie, novelization & comic adaptation: Star Trek VI: The Undiscovered Country).
Following his actions, Kirk and Azetbur reconciled their conflicts with each other's species and the Khitomer Accords went on to be signed. It was decided, also, that neither Kirk nor the rest of his crew would be prosecuted for anything they had done, which included in breaking Kirk and McCoy out of Rura Penthe and violating the Klingon Neutral Zone. However, the Conspiracy brought the an end to the crew of the Enterprise when Starfleet ordered that she be decommissioned.(TOS movie, novelization & comic adaptation: Star Trek VI: The Undiscovered Country; ST reference: Federation: The First 150 Years)
A few years later, Lieutenant Valeris- the last known survivor of the conspiracy- was recruited by Starfleet Intelligence to assist in the investigation of a recent terrorist attack on the Federation relief efforts to the Klingon homeworld, eventually revealed to be the result of attacks by a terrorist group known as The Thorn, connected to the Kriosians, a race the Klingons had conquered in the past. Their intention was to further decimate Qo'nos by detonating the remnants of Praxis, but Valeris and others were able to prevent the detonation, allowing Valeris to receive a pardon and a chance for a new start after her role in the previous conspiracy.