"Balance of Terror", written by Paul Schneider and directed by Vincent McEveety, is the fourteenth episode of the first season of the original science fiction television series, Star Trek, that first aired on December 15, 1966. It was repeated on August 3, 1967. The episode is a science-fiction version of a submarine film; writer Paul Schneider drew on the film The Enemy Below, casting the Enterprise as the American destroyer and the Romulan vessel as the German submarine.

This episode introduces the Romulans. Additionally, Mark Lenard, playing the Romulan commander, makes his first Star Trek appearance. Lenard later played Spock's Vulcan father, Sarek, in several episodes and movies, and appears as the Klingon commander in Star Trek: The Motion Picture. These roles made Lenard the first actor to play characters of three prominent non-human species in Star Trek.

On September 16, 2006, "Balance of Terror" became the first digitally remastered Star Trek episode, featuring enhanced and new visual effects, to be broadcast.

The starship USS Enterprise under the command of Captain James T. Kirk is inspecting a line of manned Federation outposts, only to find they are being destroyed by an unknown enemy. While Kirk officiates at the wedding of Lieutenant Robert Tomlinson and Ensign Angela Martine, an alarm sounds and they learn that Outpost 4, near the Romulan Neutral Zone, has come under attack.

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