I was surprised by this particular episode of the animated "Star Trek" series...it didn't suck. While the quality of the animation was horrible, as usual (and DON'T attack me for saying the obvious-- -it IS among the ugliest and worst animated TV shows ever), the story was interesting and seemed to fit into the style of the previous TV show much more than most of the animated shows.
Enterprise is looking for the crew of a scientific expedition. Their ship is in orbit but the crew beamed down to a planet and haven't been heard of since. When the usual three beam down, they find a strange planet which seems artificial because it has so many different terrains and geological features in such a small area. Only when it's too late do they realize it's a zoo...and they are the new exhibits!
The plot was clearly inspired by a "Twilight Zone" episode with a similar idea. But, the show was intelligently executed and kept my interest throughout. Now brilliant but for the cartoon series, it was a high point.