Star Trek VI: The Undiscovered Country

When Qo’noS’ moon Praxis (the Klingon Empire’s chief energy source) is devastated by an explosion, caused by over-mining, the catastrophe also contaminating Qo’noS’ atmosphere, the Klingons make peace overtures to the Federation. While on the way to Earth for a peace summit, the Klingon Chancellor (David Warner) is assassinated by Enterprise crewmen, and Kirk and McCoy are held accountable by the Chancellor’s Chief of Staff (Christopher Plummer) and sentenced to life on a prison planet. Spock attempts to prove Kirk’s innocence, but in doing so, uncovers a massive conspiracy against the peace process with participants from both sides.