The USS Enterprise is on duty near the Romulan Zone border, and things are calm enough that Kirk can preside over a wedding between two crew members. An urgent call from an Outpost interrupts the wedding, and the ship discovers that a series of outposts have been destroyed by a new energy weapon. A lucky interception of a message from an unseen space ship allows Kirk and crew to find out who they are up against: The Romulans. Using a single cloaked ship firing the most lethal weapon ever witnessed. And The ''Enterprise'' is the only one who can stop them...
This one is commonly regarded as one of the very best Star Trek episodes, of this or any other series.
!!This episode contains the following tropes:
* BetterToDieThanBeKilled: "Just one more duty to perform..."
* BigBadassBirdOfPrey: The Romulan symbol.
* BloodKnight: Decius.
* TheChainsOfCommanding: Kirk reflects on this at one point with Bones.
* DueToTheDead: The Romulan commander orders the dead Centurion's body dumped into space along with a bunch of debris to [[PlayingPossum make it seem that his ship has been destroyed]] -- but he is clearly distressed about it, and [[TalkingToTheDead asks his late friend to forgive him]].
** Scotty states that the Romulans' "power is simple impulse." This caused many people (including the writers of the ''Star Trek Encyclopedia'') to assume that the Romulans only had Impulse engines and not Warp Drive. However, considering it would take decades for the Romulans to travel space without Faster-than-light travel, it would be nearly impossible to fight an interstellar war. Eventually, this was retconned by clearly showing that the Romulans have warp capabilities in ''Series/StarTrekEnterprise''. Although ''Series/StarTrekTheNextGeneration'' DID establish that Romulan vessels were powered by artificial singularities as opposed to a warp core familiar to Starfleet. One of the EU novels, set in Captain April's era, explains this by having birds of prey capable only of sublight be deployed to their duty sectors by a warp-capable mothership. Impulse engines on Starfleet vessels are powered by fusion reactors. Knowing this and that a fusion-powered ship would have much shorter range than an antimatter-powered one, just like the Bird of Prey, it's probable that Romulan ships of the era were powered by fusion reactors and that Scott just meant that, and that artificial singularities (that would ensure pretty much unlimited range) were still to be developed.
** The ''Enterprise'' fires phasers in short bursts which detonate at a specified interval, in reference to [[RecycledInSpace depth charges]]. This could have easily been done by Photon Torpedoes had they been invented by the writers sooner.
* FantasticRacism: Lt. Stiles. From a long line of Stiles who fought in the first Romulan War. When he finds out Romulans and Vulcans look alike (and Spock himself confirms the possibility the species are related), he starts viewing Spock as a traitor. Spock saves his life during the final attack, and Stiles learns his lesson.
** The original script had Commander Hansen on the doomed outpost note that the Romulan ship was similar to Federation design (saucer and two nacelles). This fuels Stiles' tirade about Romulan spies aboard the ship. It makes more sense as this line was spoken before the big reveal of what the Romulans look like.
* GiveAwayTheBride: Scotty does the honors.
* HonorBeforeReason: The Romulan subcommander taunts the Commander into one final attack on a wounded ''Enterprise'', even though the Commander knows that his Federation opponent is too dangerous to be viewed as an easy kill.
* MauveShirt: Tomlinson, so that his death at the end will be meaningful. (And no, he doesn't wear a red shirt.)
* NotSoDifferent: "In a different reality... I could have called you friend."
* OutrunTheFireball: In this case, outrun a warp-speed plasma ball. While not fast enough, the plasma energy dissipates enough to weaken its deadly effects to little damage.
** If only their phasers had been operational at the time. StarFleetBattles assures us that every 2 points of phaser damage reduces a plasma torpedo's warhead yield by 1 point.
* OldSoldier: The Centurion
* PunchclockVillain: The Romulan Commander turns out to be one.
** The 1957 WWII submarine movie ''Film/TheEnemyBelow'' [-IN SPACE-]! Even the bearing of the unidentified initial contact and the captain's maneuvering orders to the helm are copied nearly verbatim. The Romulan ship is cramped and has lots of piping and conduits in all background scenes. The Enterprise's phasers act like depth charges, and at one point the Enterprise and the Romulan ship both go to silent running, fearing to ''talk loudly'' [[SpaceIsNoisy lest the other vessel hear them]].
** The Romulans, in this episode at least, were TheRomanEmpire [-IN SPACE-]!, as exemplified by Mark Lenard's dignified Centurion character.
* SoMuchForStealth: The Enterprise and the Romulan warbird are both attempting StealthInSpace until Mr. Spock accidentally activates a control, giving their position away.
* SpaceIsAnOcean: A bit more even than usual, what with all the submarine movie tropes.
* SpaceRomans: Mark Lenard's Romulan commander is given a hairstyle that many associate with Emperor Nero. The old warrior is called "Centurion". The subcommander is called Decius. And their ruler is called the Praetor. The old Centurion fears that Decius has friends in high places, which was also a way to climb the ladder in the Roman Empire.
* StealthInSpace: The TropeCodifier for ''Trek'', if not all of Science Fiction. And yet, an UnbuiltTrope: it is a plot point that while under cloak the Romulan ship has the same problems with detecting the Enterprise as the Enterprise has detecting the Romulan ship.
* ThatsWhatIWouldDo: Kirk about the the Romulan Commander. "He did exactly what I would have done. I won't underestimate him again."
* WaveMotionGun: The Romulan plasma torpedo.
* WeddingSmashers: Not directly, but the ''Enterprise'' is called into battle during the wedding.
* WidowedAtTheWedding: Poor Angela Martine.
** The Romulan Commander. He's only attacking Starfleet bases because he was ordered to test a new weapon (and to test the Federation's vulnerability). He'd rather not start any more wars, and just wants to go home.
** The Commander quickly recognizes Kirk as one: "He is a sorcerer, that one. He reads my thoughts."
** In a deleted scene, right before destroying the Bird-of-Prey, Kirk salutes the Commander, who replies with a solemn bow.
* YouLookFamiliar: Mark Lenard, who plays the Romulan Commander here, would go on to play Spock's father in "Journey to Babel" and a Klingon Commander in ''Film/StarTrekTheMotionPicture''.