After the abysmal critical and fan reception of Star Trek V, they had originally planned for the next film in the series to be a prequel (which makes one wonder what JJ Abrams would have done with his recent prequel reboot), but after objections from the cast and fans (and finding new pressure from fans to have new movies be about the crew from Star Trek: The Next Generation) producers decided to make a finale film of sorts with the crew of the original series set to retire making way for the new characters.
Taking themes from the front pages of current newspapers in 1991, Star Trek VI uses the fall of communism to cull out a plot about the Klingons finally wanting peace. The film begins with Praxis, a Klingon moon and home of their key energy production facility, exploding into tiny bits. Recognizing that for their entire species to survive they are going to have to get help from their old enemies, the Federation. Aided by negotiations with Spock, the Klingons, led by Chancellor Gorkon, accept a proposal.
Starfleet sends the Enterprise-A to Gorkon to escort him to Earth and further negotiations. Most of the crew of resents this mission as they've dealt with Klingons and their hostility all too often to accept peace. Kirk is especially resentful of the idea as his son was killed by Klingons (in the third film).
Animosity aside, the Enterprise-A crew meets the Klingons and bestows upon them every courtesy, including inviting the officers onboard for a hilariously tense (and drunken) meal. Later that evening (and after a scene involving Kirk recording a captain's log where he complains angrily about the mission and his distrust of the Klingons which is so oddly timed you know it will come up later in the film), all hell breaks loose. The Enterprise-A apparently launches two torpedoes at the Klingon ship disabling its artificial gravity which is followed by two figures wearing Starfleet uniforms and gravity boots who beam aboard the Klingon ship, kill several Klingons, and mortally wound Gorkon.
Aboard the Enterprise-A, no one on seems to know what is going on. The photon torpedoes appeared to come from the Enterprise-A but they weren't ordered by Kirk or anyone else, and at first they do not seem to be missing any ammunition. Likewise, no one ordered a party to beam aboard and start shooting. Kirk and Bones then do beam themselves aboard the Klingon ship to find out what is going on and to offer help. They find Gorkon awash in his own purple blood (changed from red to avoid an "R" rating.) Bones attempts to save him but being unfamiliar with Klingon anatomy fails to do so.
With Gorkon dead, the rest of the Klingon's arrest Bones and the Captain, give them a quick trial (where they are defended by ST:TNG's Michael Dorn playing his character Worf's grandfather) found guilty, and sentenced to life on a frozen prison planet.
Back on the Enterprise-A, Spock and the rest of the crew spend the rest of the film trying to determine what just happened and who really attacked the Klingons (and the scenes play out more like a generic television detective show than a big budget sci-fi movie.) Meanwhile, Kirk and Bones try to find a way to escape their prison stay (and meet a shape-shifting Iman the super-model while they are at it).
Needless to say they escape, the crew figure out who the real culprits are (rogue Klingons who can't accept peace with the Federation), and everybody races towards the ongoing peace conference to explain what really happened and to urge peace. On the way they are attacked by the rogue Klingons in a new Bird of Prey that can now fire while cloaked but are saved again by Captain Sulu and his industrious Excelsior.
Peace is restored; the crew are ordered to come home, decommission the Enterprise-A, and retire, but of course, Kirk decides to once again disobey orders and they fly off into the cosmos.
The Undiscovered Country is an even numbered film which means its supposed to be good, and it is. Mostly. It is very dated and its lower budget (due to the poor performing of The Final Frontier) shows throughout. Its pacing is brisk and light, and it's all told with a wink to the audience, as if we're to recognize this is all just in good fun. This hinders the drama a bit as we never really think that anybody is in any real danger and the mystery at the center of the film never gets teeth enough to make me really that interested. But it all unfolds fast and and with good cheer and with enough fun Trekiness to make casual and hard core fans wiling to forgive its weaknesses. All in all it's a perfectly good send off for the classic characters and makes way for The Next Generation to have its day. Little did we know then that the crews from both shows would meet up in the next film, Generations.