Devastating Defeat

The film opens with the USS Excelsior, commanded by Captain Hikaru Sulu, slammed by a shock wave traveling through space. Investigation shows that the wave came from Klingon space, specifically the moon of Praxis, the Klingons' key energy production facility. Closer examination reveals that much of Praxis has been destroyed. The Excelsior offers assistance, but are rebuffed by the Klingons. 
Praxis' destruction throws the Klingon Empire into turmoil. Without Praxis, the atmosphere of the Klingon homeworld Qo'noS will be depleted in 50 years. No longer able to maintain a hostile footing, the Empire sues for peace with the Federation. Starfleet chooses to send the USS Enterprise to meet with Klingon Chancellor Gorkon and escort him to negotiations on Earth. The decision does not sit well with Captain James T. Kirk, whose son David Marcus died at the hands of Klingons. Upon rendezvousing with Gorkon's battlecruiser, Kirk invites the Klingon entourage to dinner aboard the Enterprise. The two crews have sharp disagreements over the nature of the peace, and mutual hostility is obvious. Gorkon acknowledges relations between the two peoples have a long way to go. 
After the ceremonial dinner, the Enterprise appears to fire upon the unguarded Klingon ship with a pair of torpedoes. The torpedos score strategic hits on the ship's hull and disable artificial gravity aboard the vessel. During the confusion, two figures wearing Starfleet suits and gravity boots beam aboard the Klingon ship and attack the Klingons, wounding Gorkon. General Chang, Gorkon's chief of staff, accuses Kirk of defiling the peace process, and prepares to attack the Enterprise. Kirk surrenders to avoid a fight, and beams aboard the Klingon ship with doctor Leonard McCoy in an effort to save Gorkon's life. The chancellor dies, and Kirk and McCoy are put on trial at Qo'noS, found guilty, and sentenced to life working at a gulag on the world of Rura Penthe. Gorkon’s daughter, Azetbur, becomes the new Chancellor, and wishes to push forward with diplomatic negotiations. For reasons of security, the conference will be held on a neutral world, the location of which is kept a secret from the general public and from most Starfleet and Klingon officers. While several senior Starfleet officers want to rescue Kirk and McCoy, the Federation President refuses to risk full-scale war. Azetbur likewise refuses to invade Federation space, stating that only Kirk and McCoy will pay for her father's death. 
Kirk and McCoy arrive at Rura Penthe and are befriended by a shapeshifter named Martia, who offers them an escape route. After making their way across the frozen surface of Rura Penthe, Kirk and McCoy realize Martia betrayed them. Kirk and McCoy are about to be killed by Klingon guards when Spock, aboard the Enterprise, transports them away. While Kirk and McCoy were imprisoned, Spock assumed command and led an investigation of the assassination. Determining that the Enterprise did not fire upon the Klingon ship and that there was help on board, the crew begins looking for the traitors. The two assasins are discovered dead, but Kirk and Spock trick the traitor into believing them alive and about to give testimony. When the traitor arrives in Sickbay to finish off the assassins, Kirk and Spock discover that the killer is Spock's own protege, Valeris. Spock overpowers Valeris, performing a Vulcan mind-meld, and learns who the conspirators are (a mixture of Klingon, Federation, and Romulans). They also realize they have a prototype Bird of Prey which can fire weapons while cloaked, and was the true source of the torpedo attacks upon Gorkon's ship. 
Kirk contacts Sulu, in hopes that his former shipmate will know the location of the peace conference. Sulu tells them and both ships head for the conference at Camp Khitomer. As it nears the planet, the Enterprise is intercepted by Chang’s modified Bird of Prey. With the Enterprise unable to track his ship's position, Chang inflicts severe damage on the Enterprise and then the Excelsior when Sulu arrives. To make matters worse, Bad Vader unleashes a secret weapon! That secret weapon is a supercannon! The supercannon targets the Enterprise and the Excelsior! The supercannon charges up! The supercannon fires on the Enterprise and the Excelsior, destroying them! The entire crew of the Enterprise and the Excelsior are killed in the destruction of the Enterprise and the Excelsior. This is a devastating defeat for the Federation!