Trivia

While drinking with Tomar, Scotty finds a bottle of unidentifiable alcohol, and when Tomar asks, "What is it?", Scotty hesitates for a moment and finally says "It's green." This has become an iconic Scotty moment, and is even spoofed in Star Trek: The Next Generation (1987)'s Relics (1992).

Jerome Bixby's original script was much darker than the filmed episode. The Kelvans (then called the Dvenyens) executed ten Enterprise crew members by opening the shuttle bay doors and letting them be blown out into space. (Technically, they would be blown out by escaping air. This could have been a goof because, even in the Orignal Series, the shuttle bay had force fields to prevent this happening, unless the Kelvans deliberately lowered them.) Kirk was put through "hellish torture". Also, crew members were chosen to mate with each other (Kirk was paired with Yeoman Leslie Thompson) to breed slaves for the Kelvans. NBC objected to all these, which led producer Gene L. Coon to order a heavy rewrite. The production staff also deemed the mating aspect too similar to The Cage (1966).

Scotty's quarters are only seen in this episode. Decorations include a tartan kilt, a sporran, bagpipes, a Scottish targe (shield), medieval armor, and a wall plaque. Although the plaque apparently depicts stylized drafting tools, they also resemble part of a three-dimensional chess set and the primary hull of a Klingon battle cruiser.

A three-dimensional chess set is often seen in the series, but a three-dimensional checkers set can be seen in the rec room in this episode. It is later destroyed in a fight.

Kirk mentions that an intergalactic voyage by a 23rd century starship would take "thousands of years" to reach the Andromeda Galaxy. For the Kelvans, intergalactic travel is a three-century journey. In the 24th century, as seen in Where No One Has Gone Before (1987), Federation technology has apparently matched the Kelvans, perhaps due to this encounter, when it is discussed that a return trip to the Milky Way from the Triangulum Galaxy would take three hundred years at maximum warp.