The Federation sends Kirk to negotiate for mining rights with Aka'ar, Te'er of the Ten Tribes of Capella IV. Capella IV has deposits of the rare mineral topaline, needed for life support systems. The Klingons need this mineral, too, and their presence creates tension further complicated by conflict among the members of the tribe. Ma'ab murders Aka'ar and becomes the new Te'er, and he has a decidedly different attitude that favors the Klingonm perspective. Worse, Kirk and McCoy will not stand by while he murders Ele'en, the pregnant wife of the old Te'er, despite this being Capellan tradition. Soon enough, the Federation party is running for the hills - literally - with Capellans and a Klingon in pursuit.

On an obscure planet, Kirk smells something that reminds him of the past. And then two crewman die, their ghastly pallor betraying the fact that every red blood cell in their bodies has been destroyed, the hemoglobin neatly removed. A long time ago, very far from here, Kirk was a young Lieutenant about the Farragut when that ship encountered the cause: a mysterious, gaseous cloud that feeds on blood. Now, Kirk is obsessed with destroying the monster that killed his then mentor, the Farragut's captain Garrovik. Perhaps to an unhealthy degree. Or, given what Spock discovers, perhaps not.