Beaming down to the planet, Kirk, Spock, and McCoy encounter a wide variety of environments and strangely familiar creatures and soon deduce that they have landed smack in the middle of some sort of interstellar zoo. In short order the trio are captured by the keepers of the zoo, highly intelligent, telepathic creatures which resemble giant pink slugs, and taken to join the surviving missing researchers in the zoo's human exhibit.
An attempt to escape leads to a Latran child being accidentally beamed aboard the Enterprise. There, the youngster learns all about the Enterprise and the Federation. Upon returning to the surface, he convinces his elders that the humans are an evolving intelligent species that does not belong in their zoo. The Enterprise crew members and the rescued scientists are then sent on their way.
The writer of this episode, David P. Harmon, also contributed two episodes to the second season of the original series, "Obsession" and "A Piece of the Action," which, by the way, also begins with the Enterprise checking up on a missing starship. In addition to his work on Star Trek, Harmon enjoyed a long and varied three decade long career as a television writer that spanned from from the medium's earliest days up to the 1980's. He wrote in genres ranging from science fiction and cartoons to sitcoms to westerns and crime dramas. I find it amusing that the same man who wrote one of my favorite Trek episodes, "A Piece of the Action" is the same man who wrote the TV movies "Rescue from Gilligan's Island" and "The Harlem Globetrotters on Gilligan's Island", which,even by the admittedly low standards of Gilligan's Island, are fairly awful.
The animated Star Trek took full advantage of the freedom allowed by animation to create exotic alien creatures and landscapes, expanding the universe of Trek by opening it up to include a variety of non-humanoid aliens. Alongside the Phylosians of "The Infinite Vulcan" and the Vendorian from "The Survivor", the Latrans and their zoo complex are a prime example of this.
However, Filmation's constraints of time and budget resulted in extremely limited animation, and the "action" in this episode seems exceptionally static even when compared to other episodes in the series. Unfortunately, this has the effect this time of dragging down what is otherwise a clever and imaginative first contact tale and making it rather difficult to watch.