Enterprise visits Deneva where colonists are inexplicably becoming violent and destructive.
I enjoyed this episode for the sci-fi concept and the performances of the central characters.
It starts off intriguing and dramatic as something very strange starts to happen and there is a family connection to Kirk. From there the plot unfolds quite compellingly and there is an interesting focus on both leadership, scientific research and Spock's Vulcan physiology.
I love the idea for the single-cell creature and the connection to a host. All the science associated with this works very well for me. Another reviewer has mentioned the Cold War allegory which is good, but for me is not quite as deep as it probably tries to be.
Kirk, Spock and McCoy take centre stage in a riveting battle against an unusual enemy. This for me is a classic example of Star Trek science and logic winning the day. However, it is a bit contrived for one character to save the day when it would be more plausible that one of the other two would have provided the solution given their scientific roles. Another huge contrivance comes at the very end which by today's standards of TV show storytelling is rather poor.
The visuals are a mixed bag as the location does not look anything other than the complex in Southern California where it was filmed. However I like the creature effects, which are very simple but work well.
Performances are mostly great with William Shatner, Leonard Nimoy and DeForest Kelley all on good form, particularly Nimoy. Some of the guest actors are not so great, especially Joan Swift who is comically over the top.
For me it is a 7.5/10.