A few scenes featuring Elinor Donahue had to be re-shot, because the original film negatives were damaged and couldn't be used. Portions of the planet set had to be rebuilt, since other episodes were shot there by that time, using different sets. Meanwhile, Donahue got pneumonia and lost ten pounds. To hide this, they put Hedford's scarf around her neck and upper body. However, her weight loss is still visible on her face. The re-shots were not directed by Ralph Senensky.
This is the first story to feature Zefram Cochrane, inventor of warp drive technology and an important figure in Federation history. He would later re-appear in Star Trek: First Contact (1996) and Broken Bow, Part 1 (2001), played by James Cromwell.
A view of the starship from dead center in front of the saucer section is used only in this episode.
This is the only episode in the first two seasons in which Captain Kirk is not on the Enterprise at any time during the plot. Likewise, the Enterprise does not appear until twenty-seven minutes into the episode. In four third season shows, Kirk also spends the entire episode off-ship: The Paradise Syndrome (1968), Plato's Stepchildren (1968), Whom Gods Destroy (1969), and All Our Yesterdays (1969).
The scenes of Cochrane communicating with the Companion were all shot at one time. The set was then completely redone with his house added for all of the sequences with Kirk and company. The inconsistencies between the two versions of the same set can be seen in alien trees that are near Cochrane in one view and absent in the next.