The Enterprise encounters a giant cloud that consumes planets that lie in its path. The crew's problems are compounded when Spock discovers the cloud is intelligent.
This episode provides examples of:
- Planet Eater: The space cloud.
- 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.