The Doctor, Sarah and Harry teleports to Earth to ensure the planet is safe for the survivors on-board Nerva Beacon to return to Earth and re-inhabit their world. Only to find a Sontaran named Styre has captured a group of humans and conducting experiments on them to discover the human body's weaknesses, as part of the Sontaran's goal for domination of the galaxy.

The Doctor and his companions are sent to the planet Skaro by the Time Lords to prevent the creation of the Daleks.