"Mmmmmmmm, metal alloys."
The Enterprise encounters a giant cloud that consumes planets that lie in its path. The crew's problems are compounded when Spock hypothesizes the cloud may be intelligent.
This episode provides examples of:
- The Bus Came Back: Bob Wesley, the commodore from "The Ultimate Computer", returns in this episode as the primary authority on a planet threatened by the cloud, having left Starfleet to become a governor.
- Call-Back: When pondering potentially killing the space cloud, Kirk asks McCoy about the morality of it, bringing up his previous assertion that man learned civilization by deciding he won't kill today.
- Emergency Refuelling: As the Enterprise goes through the living space cloud it must expend an enormous amount of energy to power the shields. It eventually runs out of antimatter fuel and has to get more by removing part of the creature's alveoli, which are made of antimatter.
- Heroic Sacrifice: Kirk initially plans to self-destruct the Enterprise once the ship reaches the cloud's "brain", thus saving Mantilles and any other planets in the solar system the creature might consume. However, Spock's improvised system to mind-meld with the cloud provides another option.
- Planet Eater: The space cloud intakes and breaks up planets to absorb energy.
- Took a Level in Kindness: Possibly due to circumstances. This episode shows off more of Bob Wesley's vaunted "humanity" than his last appearance; he busies himself with evacuating as many civilians as possible and transmissions between him and Kirk are always cordial and respectful.
- What Measure Is a Non-Human?: The crew's method of dealing with the cloud relies primarily on confirming whether it's intelligent or not, and thus whether killing it is a wise action or not. The cloud had much the same problem; it had no idea that the organisms on the planet it was planning to eat were sentient because they were just so unfathomably tiny to it, that it couldn't converse with or understand them until Spock mind-melded with it.
