Getting our first glimpse at the Romulans, this episode explores themes of wartime prejudice, and the cost of conflict. The episode is framed beautifully, opening with a wedding ceremony taking place on the Enterprise between two crew members.
In great literary tradition, the wedding is interrupted. This immediately sets the episode off with a tense and unsettling mood. The literary precedence is matched early on with a mythological reference in the naming of the new race and its planets: Romulus and Remus. While the analogy has little immediate payoff in this episode, it gifts at least viewer confidence in the quality and ambitions of the episode to come.
We're also introduced to young navigator Mr. Stiles, whose family history in the century-old war with the Romulans acts as our window into the prejudicial issues at hand. His hot-headed advice is to preemptively attack, unsound advice that Kirk begins to take into consideration as the stakes get higher.
When the visage of the unseen Romulans is revealed to the crew for the first time, Stiles is shocked at the similarity they bear to Vulans, specifically Mr. Spock. Thoughout the epsiode, Stiles treats Spock deplorably, making him the object of unsubstantiated assumption and rudeness. Spock is, of course, unphased by this. The previously established viewer sympathies with Spock make way for a great moral, immediately pitting Stiles' attitude as not just upleasant, but wrong.
Spock suspects that the Romulans may be an offshoot of the Vulcan bloodline, and that their penchant for violence and war-mongering are a reflection of the un-evolved Vulcan mentality. This juxtaposition, of the Romulan's violence (not unlike many modern earth attitudes) against a higher or elevated mentality is the source of the "optimism" people reference when talking about Star Trek. The idea that violence and war are merely a phase in the adolescence of a culture is inspiring.
Then there's what's happening with Kirk. We open with him comfortably, and honorably, officiating a marraige ceremony. This is a classic aspect of Captaincy, and get's us ready to deal with other long-laid issues of every type of command. At the brink of war, Kirk must face his match: a command intellect equal to his own. The showdown is a sight to see, and, for the time, quite spectacular. At it's zenith, and Kirk's victory, the Romulan commander says "You and I are of a kind." It was as if Gods were dueling.
The conflict puts our Mr. Stiles in a dire situation, which Spock unflinchingly saves him from. This act of instantanious compassion, from a man he'd gone to great pains to despise, transformes him. His attitude is corrected, a nobler mindset inhabited. But this action-packed conflict also brought an unfortunate casuality: the death of the episode's groom-in-waiting.
In the final scene, Kirk consoles the mourning bride. Through his comforting words, "there was a reason ... " Kirk finds himself immobilized. Holding on a final introspective look at Kirk, we see that he struggles to believe this statement himself.