With a name inspired by the 1968 "Pueblo Incident", where the Navy ship Pueblo strayed too close to North Korea and was seized by its communist government, this espionage episode is like a James Bond story, with action, romance, a disguise, and a twist... but with the pieces assembled in a way that plays against expectations. For once, Nimoy gets to play the romancer while Shatner gets to wear the pointy ears and upswept eyebrows! In fact, this one is really a Spock episode, with Kirk's McGuffin chase clearly secondary.
Joanne Linville guest stars as the Romulan Commander, bringing a Kate Mulgrew-like sexiness to an older and more commanding female part than TOS usually includes. (Linville was forty when the episode was She and Nimoy (three years her junior) work well together, setting up a chemistry between Spock and the Romulans that works well for nearly fifty more years.
In fact, everyone seems to have a good time, making it easy to overlook how terrible "the good guys" are acting when you look at it from the perspective of a foreign government. But the joy of this episode isn't in the story but the teleplay, which is packed with emotion and excitement. (There's even a new score by Alexander Courage, the composer of the TOS theme song, to heighten the drama... though he overdoes his staccato hits). And despite being contrary to the usual values of Starfleet and Gene Roddenberry (or maybe because of it), the offbeat episode is a fan favorite that often cracks top ten lists.
Remastered Verson: CBS greatly improves this episode, with the CGI ships capable of telling the story much better than the original models. This, of course, is the episode that establishes that the Romulans have borrowed (or stolen) Klingon ship designs, which was necessary after the original Romulan model was lost as well as a cost saving decision that allows this episode and "Elaan of Troyius" to share effects. (Interestingly, these two episodes in their original form represent the first time these Klingon ships are seen up close. The remastered project, of course, sprinkles them into the first two seasons). The upgraded version of "Enterprise Incident" not only has the Klingon ships referenced in the dialogue, but throws in some classic Romulan ships as well, giving us the best of both worlds.