This is the Saturday morning cartoon version of Star Trek, which was made a few years after the original series was cancelled. It features all of the regular cast members except Chekov, who was replaced with an alien character.
It lasted 2 seasons, 1973-1975. All the episodes are available in a single DVD collection.
The original cast members provide their voices, and many also provide voices for 'extra' characters. James Doohan voices 4 extra characters plus Scotty in one episode! Nichelle Nichols does quite well with extra voices, too, but Majel Barrett's voice is too distinctive to really work well for extras.
Some consider the Animated Series to be Star Trek's 4th season. Third season had banned 'comedy' episodes, but the Animated Series lets them back, so you get a Tribbles sequel and another Harry Mudd episode. (Also a sequel to Shore Leave, but not a comedy.) The episodes are shorter, but due to the faster pace of a cartoon, there is almost as much content as a full-size show.
Due to the action-oriented nature of cartoons, some characters get more airtime than they did in the original series. Scotty – looking very buff! – almost replaces Bones as the third major character. Uhuru gets to command the Enterprise and save Kirk!
The series intentionally takes advantage of the format to show things you could not do in the original series, from fantastic aliens to Vulcan planetscapes, from giants to miniaturized people. You see new ships, and even an aquatic shuttlecraft.
There are also changes to appeal to a Saturday morning audience, with more 'silly' humor and less violence. Lots of running. One episode won praise for delicately handling the subject of euthanasia for pets, yet another episode where an entire ship plans suicide seems to not have bothered the censors!
Unfortunately, the cartoon is just dreary to watch, and I'm hard put to explain why. Certainly, using all-new music seems to be a mistake. Perhaps DC Fontana, who shepherded this series, was out of her depth here, or didn't 'get' the format (but she kept the characters true to form). The actors, especially in the early episodes, sound unnatural and almost sleepy.
Recommended for completists only. It is nice to hear the original cast one more time, I only wish they had figured a way to master the format for Star Trek.
And why are they Oh-REE-on pirates?!