The theme of the song played through the flashback sequences near the end of the episode.
It was written by Dorothy Fields and Jerome Kern in 1936, originally performed by Fred Astaire. Among the artists who later covered it were Bing Crosby and Frank Sinatra.
The authors of the Star Trek Encyclopedia (4th ed., p. 2482) described the song as expressing the emotions of love which were associated with the visual appearance of one's mate. Furthermore, it was stated that Vic Fontaine's performance of this song was the final scene filmed for the series.