Synopsis: Ambassador Sarek comes aboard the Enterprise with his staff and new, human wife, to complete treaty negotiations with the Legarans who insisted on him mediating. Soon after his arrival, the crew of the Enterprise begin to suffer from bouts of violence and other emotional outbursts. Eventually, the crew discovers that Sarek is suffering from a rare Vulcan disease that erodes all emotional control in Vulcans. Since Vulcans are highly telepathic, he has been broadcasting emotions to the crew. But, because of his illness, the important treaty with the Legarans is in jeopardy.