I'm not sure if I should tell this story or not. On the one hand, it's nobody's business. On the other hand, Grace Lee Whitney was a good person with a hard life, and this is a good illustration.
I never met Grace Lee Whitney, but I was friends with one of her old boyfriends from the '60s, and someone who worked on TOS. I'm not going to name my (living) source because they'd take flack and they're old and don't need this crap, so if you want to say I'm lying, that's cool. Anyway I was told why she got fired midway through the first season. There were basically two reasons.
Part of the problem was that she wouldn't sleep with Roddenberry. He was notorious for using the casting couch. That was kind of taken for granted in those days, but evidently he was so flagrant in his use of it that the studio had to tell him to knock it off, or at least close the door. She was dating my friend, and she wasn't the cheating kind, so she refused Gene. This was a strike against her, but it wasn't what actually got her fired.
What actually got her fired was Majel Barrett. Majel reasoned that there were three women characters on the show - Chapel, Uhura, and Rand - and therefore if the script called for a female presence she basically only had a 1/3rd shot at any given time. However, if she could get the other women fired, then any script calling for a female crewmember would mean she got lines.
Well, she couldn't get rid of Nichelle, becuase they'd already gotten so much good press for having a black woman. She could, however, push Grace out without too much trouble, and that would at least increase her odds to 50/50 script-wise. So she campaigned against Grace. She was Gene's main girlfriend, and whispered in his ear a lot. I don't know what happened beyond that, but between her pull with him, and Grace's refusal to sleep with him....boom. She was out.
And that's that.