A cloud-like creature devours planets and heads toward a populated world.
A giant cloud that consumes planets heads for Mantilles, a Federation colony of more than eighty million people governed by retired Starfleet officer Bob Wesley. Phasers have no effect, so Kirk takes the Enterprise inside the cloud, where the ship loses power and Scotty draws antimatter from the cloud to keep the engines running.
Spock discovers that the cloud is a living being with a brain. Instead of destroying it, he makes mental contact through the ship's sensors and lets it perceive that the worlds it eats hold life. The creature agrees to spare Mantilles and returns to where it came from. Veteran Star Trek director Marc Daniels wrote the episode.