The Enterprise is transporting an ambassador who will be organizing peace talks between Pavak and Oyolo, two warring planets. The ambassador in question is Kevin Riley, former crewmember of the original Enterprise. Meanwhile, Spock and Scotty are on Pavak overseeing the disarming of the Pavakians' protomatter missiles. They find themselves detained when news of the assassination comes through but discover that one of the warheads was removed prior to destruction.