The Enterprise encounters a powerful energy force that has apparently killed all human life in a solar system with over one billion inhabitants. They identify the culprit as a small space probe that had its origins on Earth. Called Nomad, it merged with another and, as a result, took on a new mission to destroy all biological beings as being imperfect. It believes Captain Kirk to be its creator and, as such, has spared the Enterprise and its crew, at least temporarily.
Did You Know?
Per Irish mythology, a "changeling" is a demon child substituted by the spirits for a human child they have stolen. That is the context used here. However, the word can also mean a shape shifter, as in several other contexts within the Trek Universe. The most famous shape shifter changeling in Trek was the regular character Odo from Star Trek: Deep Space Nine
(1993). See more
When Spock, Kirk and McCoy view a diagram that shows the various parts of the NOMAD probe, the lower right of the diagram contains the annotation: "Computer Data Banks Recall & Transmition," should read "Transmission". See more
[outtake from a scene shortly before beaming Nomad aboard the ship
We are prepared to beam you aboard.
Captain... you forgot all about the environment and all that stuff. Do you wanna really do that?
[everyone bursts out laughing