This review contains spoilers.
After the dire "Spock's Brain", things pick up considerably with this great instalment, which probably stands as one of the third season's best.
This episode would have made a much better season opener than "Spock's Brain"; it has an infinitely better story, and a strong Spock plot – one of the reason's "Spock's Brain" was selected to air first.
Anyway, Kirk is acting erratic and tense, but from the outset you know that there's more to the situation than that. As it turns out, it is a case of a Federation plot to steal a Romulan cloaking device. The plot unfolds very well, and has several good twists.
For a Romulan, the Commander (Joanne Linville) is very appealing, and Linville plays the part both powerful and seductive at the same time.
This is also a good episode for Mr. Scott, who has several good moments to shine. Scotty is one of my favourite characters, and it's good to see him have his moment in the spotlight.
As is very often the case, the Enterprise comes across new technology that is never used again; in this case, they secure the Romulan cloaking device, and hook it up to the Enterprise to make their escape, but the technology is not incorporated into subsequent episodes. One must assume that it did not meld well with Federation Starships!
One thing I will say is that the preview trailer, featured (as with all episodes) on the DVD release, does spoil the story somewhat, and if you are going into the episode without knowing the outcome, I would strongly recommend not viewing the trailer until afterwards, so as not to spoil the story.
All-in-all, this is a very good episode by third season standards, even if you do have to suspend your belief at a couple of points. It doesn't quite make a perfect 10 for me, as there are a couple of spots where the story sags slightly, but it is still an extremely good episode. It's a shame more of the third season episodes weren't of this standard.