Responding to distress calls from border outposts along the Neutral Zone between the Federation and the Romulan Star Empire, Kirk and the crew receives a final message from a Federation station reporting an attack from an invisible ship, now running at full speed towards home. The U.S.S. Enterprise pursues the warbird, hampered by the fact that the Romulans have constructed an invisibility screen which shields them from view. After every attempt to lose the Enterprise fails, the Romulan Commander is forced to turn and fight. Both ships are damaged and rather than let himself and his ship be taken prisoner, the Romulan Commander self-destructs the warbird.

Mark Lenard (Romulan Commander)
Paul Comi (Lt. Andrew Stiles)
Lawrence Montaigne (Decius)
DeForest Kelley (Dr. Leonard H. McCoy)
Grace Lee Whitney (Yeoman Janice Rand)
George Takei (Lt. Hikaru Sulu)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)
Stephen Mines (Lt. Robert Tomlinson)
Barbara Baldavin (Specialist Angela Martine)
Garry Walberg (Commander Hansen)
John Warburton (Centurion)

• This is the first appearance of the Romulans, who look very much like Vulcans. Spock deduces that they must be an offshoot of the Vulcans, from the time when they were aggressive and savage.
  
• The Neutral Zone was established 100 years prior, after the Earth-Romulan War. They had never seen a Romulan until now.
  
• The Federation built outposts along the Neutral Zone, constructed on asteroids.
  
• The Captain was in the middle of a wedding ceremony between Tomlinson and Martine when the ship went to red alert. Tomlinson died during the attack.
  
• Mark Lenard will later play Spock's father Sarek in the episode "Journey to Babel".
  
• This is the last appearance of Janice Rand. She will return to the Enterprise in Star Trek: The Motion Picture. (Apparently, Grace Lee Whitney was assaulted on the studio lot by a Star Trek executive. She didn't report the attack, but she was soon fired from the series.)