This episode introduces the Romulans, who, along with the Klingons, are the main antagonists to the Federation. It begins with Kirk officiating a wedding, with Scotty giving the bride away, but the ceremony is interrupted by a klaxon. Someone is attacking outposts along the Neutral Zone, a DMZ between the Federation and the Romulans after a devastating war a century earlier.
The culprit is a Bird of Prey, a highly advanced craft commanded by an unnamed Romulan played by Mark Lenard, who would go on to portray Spock's father.
The rest of the episode plays out like Das Boot in space, with Kirk and the Romulan locked in a battle of wits. Naturally, the Enterprise prevails, and the show ends with a rumination on war. Before the Bird of Prey self-destructs, the Romulan commander says to Kirk, "In a different reality, I could have called you friend."
