The role of Joan Conway (the Alphan in two scenes in the Nuclear Generating Area) was played by an uncredited Rula Lenska, who became a source of ridicule in the U.S. when she starred in a series of shampoo commercials in the late '70s, tagged as a "World Famous Actress", when in fact she was virtually unknown in the States.
This was the tenth episode in the series' production order and the first to feature the spaceship model work of Martin Bower. The ship belonging to Jarak's people was created for the episode in two scales (the larger one nearly two meters in length). The pursuing ship that comes in at the end of the episode was an older model Bower had built (a meter and a half long), which was partially inspired by the Discovery, the spaceship sent to Jupiter in 2001: A Space Odyssey (1968).
Julian Glover (Jarak) is married to actress Isla Blair, who had guest-starred on the episodes War Games (1975) and Journey to Where (1976). The pair actually played husband and wife in Indiana Jones and the Last Crusade (1989), with Glover playing the movie's main villain, Walter Donovan (who "chose poorly").
This episode was originally written by Edward Di Lorenzo (who had written Ring Around the Moon (1976) and Missing Link (1976)) from a story idea by Gerry Anderson (conceived during the series' preproduction) but was completely rewritten by series Story Consultant Christopher Penfold. Filmed July 8th to the 19th 1974, with the exception of the Nuclear Generating Area scenes, which were shot July 5th during filming of Force of Life (1975) to use the Generating Area set from that episode.