In case you forgot: The Enterprise is on a rescue mission, and one of Kirk's old flames is in the mix. Turns out the rescue mission is a ruse that Kirk's angry ex cooked up so she could use old alien technology to bodysnatch Kirk and be the captain of a starship she always wanted to be (but can't because she's a woman, and it's hard to tell if she started out unstable or if being denied her dreams and goals made her that way), while Kirk himself is stuck being....a woman. Some court martial procedural stuff happens, Kirk gets back in his own body, and justice is served.
This is one of the weirder episodes of TOS when it comes to feminism: you can easily read it as condemning the damage that happens when you needlessly limit women's roles in society. You can also read it as a condemnation of women who ~want it all~ and can't be happy with the lot of "beautiful love interest" assigned to them. Considering the source, the intended message is probably the latter, but I'll still choose to interpret it as a sci-fi statement about the importance of women's lib.
All the same, it's another bodysnatchers episode, and again it's frustrating that after all this time it's not immediately apparent to everyone that Kirk is not himself. Also, watching Shatner act out being possessed by a woman is one of the most eyeroll-inducing performances he's given.
Memorable line: "Believe me, it's better to be dead than to live alone in the body of a woman." (I feel like Roddenberry meant this in a very different manner than I'm taking it, but still, it stayed with me.)