Script editor Robert Holmes was not a fan of six-part stories, believing that they were padded, so for season twelve, he decided to have one four-part story and one two-part story. With Christopher Langley's "Space Station" already in the works, he looked to Bob Baker and Dave Martin to fill the gap. As an outline, Holmes asked them to set it on Earth and bring back the Sontarans. This would help them capitalise on the costly ship interior and Sontaran costume used in The Time Warrior.