Star Trek: Season 2, Episode 13

2 - 13: Obsession

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.