Deleted scenes: Lt. Joan Harrington attempts to help Roper pack his belongings in the Moonbase Sleep Sphere; Roper passes through a Radiation and Documentation check at Earth Spaceport.
As UFO was shown out-of-order, this was actually the third episode produced.
This was Maxwell Shaw's third appearance as Dr. Schroeder (the three episodes being among the first to be filmed), but heart surgery intervened, causing only one more appearance after recovering (E.S.P. (1970)).
This episode's original script title was "The Sun Also Rises" with the character of Paul Roper named Paul Bridges. Also, instead of the aliens using a SHADO go-between as Roper's contact, it was an actual alien.