Near the end of the episode, Scotty tells Kirk that the ship's dilithium crystals are deteriorating.
Kirk asks "Time factor?"
Scotty replies "In 12 minutes we'll be totally without engine power."
This happens with exactly 12:00 minutes left in the episode.
This happens with exactly 12:00 minutes left in the episode.
Mara is the only female Klingon with a speaking role on TOS. Another female is beamed on board the Enterprise at the same time, but only seen briefly.
The spinning alien entity effect was created with a child's spinning windmill bought from a stand at Santa Monica beach. Visual effects supervisor James Rugg was told to be creative as the production budget for season three had been drastically cut. He bought it after noticing how it glistened in the sunlight and filmed it against a black velvet cloth from several different angles, deliberately out of focus, turning with the help of a desk fan and with different gels on the spotlights. To help make it feel unearthly and disguise what it was, he also under-cranked the film in the camera to make it appear to spin faster and ran the film backwards.
This episode was originally written with Kor from Errand of Mercy (1967) as Kirk's Klingon adversary. Although John Colicos wanted to reprise his role, he was in Europe making Anne of the Thousand Days (1969) and was unavailable. The part was recast with Michael Ansara as "Kang".
Although intra-ship beaming is routine in later incarnations of 'Star Trek' (where it is called "site-to-site transport"), this is the first and only time it is done in the original series, although it is also referenced in Shore Leave (1966) when Sulu notes that Spock is beaming down "from the bridge".