A Klingon warship jumps the Enterprise en route to a diplomatic reception, convinced the Federation has been secretly attacking Klingon vessels, and once Kirk talks his way to a truce, he learns ships from several other species have been vanishing the same way. Running the disappearances to ground means working with the very fleets that just tried to kill him, before the pattern drags everyone into a war nobody actually wants.
Summary by Loreful, from public sources