The original outline involved crystalline space parasites, whose queen could mimic the sound of a distress signal in order to draw spaceships to their doom. The parasites could leap from vessel to vessel, and their gaze allowed them to influence people's thoughts. When the Doctor and Bill arrived, several ships, including some that were at war with each other, had been lured to the parasites' nest. As the parasites picked off the ship's crews one by one, the survivors were forced to make difficult decisions due to their limited reserves of oxygen. Another source of drama was the paranoia which stemmed from the uncertainty over which individuals had been brainwashed by the parasites. There would also be a climactic conflict about whether the parasite queen should be allowed to survive. Considering the original brief,
Jamie Mathieson decided to reemphasise the theme of space as the enemy. Instead of the mind-controlling parasites, the ships would now be plagued by their own dead, who had turned into zombies after being exposed to the absence of air pressure in the vacuum. Forming an uneasy alliance with a group of survivors led by Kline, the Doctor and Bill found a variety of ways to navigate between the different vessels. The Doctor eventually discovered that the zombies were being created by a crystalline entity living in a nearby asteroid, which he convinced to cease its attack.