Nicholas Courtney and John Levene reprised their roles as Brigadier Lethbridge-Stewart and Sergeant Benton respectively. Levene had started his role with the Second Doctor story "The Invasion" (1968) as a member of the military organisation UNIT (the United Nations Intelligence Taskforce). Courtney started earlier in the same year in "The Web of Fear", with his character's rank being a colonel. They, along with Sladen, would be the transition cast to carry through from the Third Doctor to the Fourth Doctor, though this would be the only UNIT story for the twelfth season. The Earth-based stories involving UNIT, which had regularly featured in the Third Doctor's period, were introduced partly as an effort to reduce production costs when the series moved into colour by Peter Bryant and Derrick Sherwin, the show's previous producer and script editor, as well as to base the series more on "The Quatermass Experiment" (1953).