Kirk, Bones and Spock visit a planet to obtain a cure for Rigellian Fever.
I found this to be a mixed bag of an episode that starts off painfully slow but gets more interesting as it goes on.
Many writers have observed it's connections to the movie 'Forbidden Planet' and indirectly Shakespeare's 'The Tempest', but it also contains plot elements used in other Star Trek episodes such the super-powerful character, the female companion, and the connection to real-life Earth history.
I enjoyed the latter half of the episode when the away team discover the truth about Rayna. There are a number of compelling moments, such as her becoming self-aware and having deal with emotions regarding both Flint and Kirk. Plus Flint witnessing and reacting to his master plan backfiring is good. My favourite scene and something I did not expect was the final scenes involving Spock and Kirk.
That being said the notion of Kirk falling for Rayna in such a big way is as implausible as most romance plots written in Star Trek. In fact you might be forgiven for wondering if Spock used this same technique after the events of 'City on the Edge of Forever', 'The Paradise Syndrome', and 'Elaan of Troyus'.
As a character I can take or leave Flint. I think the psychology around creepy father-daughter-future-lover relationship with Rayna is good, as is the idea about the use of Kirk to his advantage. However, the idea of him within Earth's historical context as Brahms, Da Vinci etc is a bit silly. James Daly is pretty forgettable in the role.
Generally the cast is fairly good with the three stars all solid and Louise Sorel decent as Rayna.