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.

Under the terms of the Organian Peace Treaty, Sherman's Planet will become a colony of whoever can best develop it. To that end, the Federation intends to ship a large quantity of quadrotriticale to that planet, and Federation Undersecretary Nilz Baris is determined that the grain will arrive, so he orders the manager of Space Station K-9 to issue a distress call! Enterprise responds to discover no emergency; Baris just wants guards for his grain. And then a Klingon ship arrives, seeking shore leave. And Cyrano Jones, an itinerant trader, also arrives, with some odd animals he calls tribbles. Some very hungry, odd animals...