The Doctor, Sarah Jane, and Harry Sullivan teleport down to what they believe is an uninhabited Earth from Space Station Nerva. The colonists above need it to re-colonize Earth, but it is misfiringmisfiring, and the Doctor begins to work on the system. The other two explore, but Harry falls into a trap and Sarah goes to find the Doctor. She runs into a desperate astronaut named Roth (Peter Rutherford). He has been captured by a robot and tortured by an alien. He takes Sarah close to the other astronauts, but will not go near them, suspecting them of colluding with the alien in some way.
The Doctor has been captured by three other astronauts. One of them fell of a cliff running from the robot and the Doctor was found over his body. Naturally, they accuse him of murder. They do not believe his story, as Space Station Nerva is just a legend to them. They have returned to Earth to investigate a distress call, but their ship was vaporized, leaving nine of them stranded, and they have begun to vanish one by one.
Roth appears and they chase after him. Sarah frees the Doctor, and they meet up with Roth. The Doctor also falls down a crevasse and the robot returns, capturing Roth and Sarah and bringing them to the alien’s spacecraft, a metal sphere. The alien is Field Major Styre of the Sontaran G3 Military Assessment Survey (Kevin Lindsay) at the behest of the Grand Sontaran Strategic Counsel. Sarah has met a Sontaran and is terrified. He has been experimenting on and killing the astronauts to learn how much pain and stress human beings can stand, because the Sontarans are thinking of invading Earth. Roth runs, rather than submit to more torture, but is shot dead. Sarah, as a female human, is too interesting to kill.
Styre reports via radio link to this Marshal (Also played by Kevin Lindsay). The Sontaran fleet is waiting for completion of the report. Styre reports that humans are weak and vulnerable, but he is obsessed with details and needs to do more torturing. Styre subjects Sarah to terrifying hallucinations. The Doctor, having climbed out of the hole, finds her and rips the hallucinogenic device off her head. SheThe falls unconscious. The Doctor attacks Styre in a rage, but is shot by Styre, who thinks him dead.
The robot has captured the three remaining spacemen and Styre begins torturing them. The Doctor, recovering, disables the robot, finds Sarah and Harry. He goads Styre into hand-to-hand combat while Harry sabotages Styre’s ship. Styre, low on energy, makes his way back to his ship to recharge his body, but the shipit kills him. The Doctor informs the Marshal that Styre’s mission has failed and the Sontaran invasion plans are now in human hands. This cancels the invasion and the three time-travellers attempt to return to Nerva as the astronauts have done.
The story was filmed on Hound Tor on Dartmoor. Tom Baker tripped and cracked his collarbone on location. Everyone gathered, worried, at the local pub, but Tom made his grand entrance, wearing a sling. He used his scarf to hide the neck-brace, and he was doubled in action scenes by stuntman Terry Walsh.