In this movie sulu spots a klingon moon that has exploded causing shockwaves which hit the now commisioned USS Excelsior which sulu commands. Sulu places a call to the klingon empire asking if they need assistance.

the klingons respond with "we have everything under control. we have no need for assistance. please obey neutral zone treaty this transmition ends now"

Sulu reports this happening to the federation. Sarek suggests to Spock that a dialogue be opened to Chancellor Gorkon of the klingon high command. Spock personally vouches for the crew of the enterprise to meet with chancellor gorkon but there is a conspiracy among members of the federation & the klingon empire to assasinate gorkon and blame his assasination on Kirk & Mccoy