Budgetary and time constraints prevented the make-up and costuming departments from dressing up each Romulan in Vulcan ears as it was such a lengthy process applying them. So they hit on the idea of giving the lesser Romulans helmets, which were manufactured by Wah Chang. Mr. Chang was responsible for creating many iconic Star Trek hand props.
First appearance of the Romulans in the series.
Mark Lenard plays the Romulan Commander, an apparent enemy of the Enterprise and its crew. However, later in his career he played the famed role of Spock's father, Sarek, and also played a Klingon in Star Trek: The Motion Picture (1979), making him the first actor to portray the three major alien races (Vulcan, Romulan, Klingon) in the Star Trek franchise.
The plot of this episode is based on The Enemy Below (1957), with the Enterprise taking the part of the American destroyer and the Bird-of-Prey with its cloaking device taking the part of the submarine. This episode was also partly inspired by another World War II submarine drama, Run Silent Run Deep (1958), which was directed by Robert Wise, who would later direct Star Trek: The Motion Picture (1979).
When Leonard Nimoy held out for a better contract after the first season, Mark Lenard and Lawrence Montaigne were the two leading candidates to replace him as Spock. Nimoy eventually got a raise from $1,250 to $2,500 per episode.