Three of the cast members previously guest starred on Star Trek (1966): Glenn Corbett (Paul Morgan) as Zefram Cochrane in Metamorphosis (1967), Susan Howard (Donna Krebs) as the Klingon Mara in Day of the Dove (1968), and two-time guest star Morgan Woodward (Punk Anderson) as Dr. Van Gelder in Dagger of the Mind (1966) and as Captain Tracey in The Omega Glory (1968).
This marks the first time in which the events of three episodes in a row follow each other without any time in between.