The new crew member Lieutenant Arex appeared in the first animated episode by release order, Beyond the Farthest Star, but did not get a line and was not referred to by name until this episode. He replaces Ensign Chekov at the navigation console, presumably because Chekov transferred to another ship (in fact this is explicitly the case according to startrek.com's biography).

The production reason was that although the producers paid for voice acting from William Shatner, Leonard Nimoy, DeForest Kelley, James Doohan, George Takei, Nichelle Nichols, and Majel Barrett, they wouldn't extend their budget as far as Walter Koenig. So poor Chekov got the shaft.

Transcript

1
Captain's log, Stardate 5371.3. A giant cloud has swallowed Alondra and is bearing down on the inhabited planet Mantilles.
Kirk: Intercept course!
2
Arex: We've blundered straight into the cloud and been swallowed.
Kirk: Thank you, Lt Arex. Spock?
Spock: A most skilful manoeuvre, captain.
3
Spock: This cloud is from outside our galaxy, and is... alive. It eats planets to survive.
Kirk: It'll eat Mantilles!
Spock: Yes. In 31 minutes.
4
{exterior shot of Enterprise inside the cloud}
Kirk: Can you communicate with it, Spock?
Spock: Impossible, captain. It requires physical contact. But... I may be able to do it remotely.
Caption: Almost actual dialogue.
5
{Spock concentrates}
Spock: Cloud, we are small beings. You must not consume planets. Millions will die.
Cloud: {telepathically} Do not wish to kill.
Spock: Return to your origin. Consume planets there.
Cloud: {telepathically} Okay.
6
Kirk: Imagine being so big that you can't even perceive the millions of tiny beings you kill whenever you eat something.
SFX: crunch! {bites an apple}