Star Trek VI: The Undiscovered Country plot summary
The Klingon moon Praxis has exploded, and the Klingons face an ozone crisis that they can't overcome without the Federation's help. Spock volunteers Kirk and the soon-to-be-retired crew of the Enterprise to go meet with the Klingon ambassador. But an explosion on the Klingons' ship kills the ambassador, and as Enterprise is the only ship around, Kirk and McCoy are arrested and taken to Rura Penthe (a hard-labor camp on a frozen world). The Enterprise and Sulu's Excelsior then race to save the two from lifelong slavery, while working to intercept an assassination attempt on the former ambassador's daughter at a peace conference.
