Helena's line, "We know immortality is possible," refers to the Alphans' previous encounter with the Earth survivors in Death's Other Dominion (1975); a rare reference to an earlier episode other than the first, Breakaway (1975).
There's a deleted scene where computer reports Baxter's death on Helena's medical monitor. Another deleted scene shows a longer version of Balor being wheeled into Medical (both seen in the "This Episode" sequence).
Story Consultant Christopher Penfold was unhappy with this story. ITC in New York was pressuring the writers for episodes featuring a "monster of the week" formula. In fact, there was so much friction between Penfold and ITC New York during this episode's production that Gerry Anderson (who habitually took ITC's side, as he felt beholden to them) asked Penfold to step down as Story Consultant for the remainder of the series. Penfold then became simply a staff writer with Johnny Byrne taking on the Consultant role.
One of the seven series episodes to be adapted by Power Records in the mid-70s as an audio drama for children. The basic plot of the episode remained unchanged, though it is strongly compressed, with the Baxter subplot given extremely little time by comparison to the televised version.
Filmed October 9th through the 23rd 1974.