Undoubtedly one of the best episodes ever, Balance of Terror is 45 minutes of well executed suspense, with intelligent real-world parallels (the title refers to a situation very similar to what was going on between the US and the Soviet Union during the Cold War), spot-on characterizations and the introduction of Star Trek's second most important hostile alien race after the Klingons: the Romulans.
After receiving a distress call from a Federation outpost, the Enterprise is dangerously close to the Neutral Zone which, if crossed, would lead to open conflict with the Romulans, although no one has ever actually seen them in the flesh. Soon enough, a Romulan vessel appears, carrying a new weapon and a cloaking device which makes it nearly impossible to defeat. Facing the threat of imminent annihilation, Kirk must engage in a battle of wits with the Romulan Commander (Mark Lenard) to ensure the survival of his crew. Unfortunately, the task is made more difficult when one of the men accuses Spock of being in league with the enemy, due to the physical resemblance between Romulans and Vulcans, two races that are, in fact, distantly related (a fact that is quite ironic with hindsight, given Lenard went on to play Spock's father Sarek starting with Season 2).
Always very critical when it came to the subject of war, Star Trek enjoys one of its finest hours with its most gripping and tense take on the topic. Although the Romulans aren't actually based on the Soviets (the name is actually taken from Romulus, the founder of Rome), the scenario is quite obviously inspired by the very vivid fear American and Russian citizens had at the time that either nation might be able to destroy the other with nuclear weapons (that fear gave birth to the titular concept of "balance of terror"). But even without the subtext, this remains an essential episode, due for the most part to the intellectual battle between the two adversaries, which translates into a thesping duel between Shatner and Lenard. No need to say who wins...