Killing off Villa and Jenna in this episode was considered. Villa remained because a survey showed him to be the series' second most popular character. Jenna, on the other hand, was written out. Chris Boucher realised that Gan's death had caused upset among viewers, so he decided not to kill her off.
Terry Nation originally wanted the invading alien force to be The Daleks from Doctor Who (1963), which he had created. Tom Baker and Gareth Thomas actually wanted to have a crossover between the two series. Chris Boucher and David Maloney weren't in favour of this.
Last appearances by Sally Knyvette as Jenna Stannis, Brian Croucher as Travis, and last regular appearance of Gareth Thomas as Roj Blake.
Due to BBC rules at the time, Producer David Maloney could not be given a director credit so there is no director credited on-screen.