"Balance of Terror" is still one of the best Star Trek episodes ever made. It was inspired by the film "Enemy Below" (starring Robert Mitchum), a movie that deals with a cat-and-mouse game between two captains during World War II. In this episode, Captain Kirk and his crew play similar game with a Romulan vessel. This is the program that famously introduces the war-hungry Romulans, who are distant relatives of the Vulcans. It is an incredibly suspenseful episode, tightly constructed for maximum effect. It is also interesting to see how the episode contains a series of subplots that add extra layers of meaning to the story. Mark Lenard makes his Star Trek debut as the Romulan Captain (he will later play Spock's father Ambassador Sarek). A must-see episode!