By the Transmat Circle

After defeating the Wirrn (see the previous story "The Ark in Space") The Doctor and his two travelling companions, Sarah Jane Smith and Harry Sullivan, transport down to Earth from The Ark so that The Doctor can carry out repairs to the teleportation device. They arrive on what they expect to be a desolate and deserted Earth only to discover that a group of shipwrecked astronauts, from a human colony, GalSec, have been lured there by a fake distress call.

As The Doctor begins repairing the teleportation device, Sarah and Harry explore the surrounding area, but Harry falls down a crevasse forcing Sarah to seek The Doctor's help but when she returns to the teleportation device he is nowhere in sight.

Sarah returns to the crevasse that Harry had fallen down but discovers that he too is nowhere to be seen. Roth, one of the shipwrecked astronauts, finds Sarah. He is obviously distressed, and explains that he is on the run after being caught by a patrolling robot and then tortured by an alien that lives in the rocks.

Doctor Using His Sonic Screwdriver

Roth takes Sarah towards the astronauts' campsite, but he refuses to approach the campsite, suspecting the astronaut Vural of collusion with the alien. In the camp The Doctor, who has been captured by three of the astronauts, is being blamed for the disappearance of their crewmates. The Doctor tries to explain about The Ark but they believe that Space Station Nerva to be a legend and so do not believe him.

Roth then appears and the astronauts chase after him, while Sarah frees The Doctor. Roth loses the others and meets up with Sarah and The Doctor near the crevasse. On learning about Harry’s disappearance The Doctor inspects the edge and discovers it to be a trap when he too falls down the crevasse. Then without any warning the patrolling robot arrives and captures Sarah and Roth.

The Doctor, Sarah and Harry

The Robot takes Sarah and Roth to the rocky area where Sarah is shocked to discover that the alien is a Sontaran. Roth tries to escape but is shot dead by the Sontaran. Much to Sarah's horror she discovers that the Sontaran, Field Major Styre, is conducting deadly and gruesome experiments on the humans so as to assess the human race as an adversary. He is compiling a report on human physical and mental capabilities prior to a full scale invasion of Earth.

Styre reports back to his Marshal via a video link. The Marshal is impatient for the intelligence report, without which an invasion of Earth cannot take place, but Styre admits that he has been delayed in his experiments.

With the added pressure imposed by his superior, Styre increases the experiments and chooses Sarah as his next victim. After being tied up and subjected to one of Styre's experiments, which involves a series of terrifying hallucinations, Sarah is eventually rescued by The Doctor who discovered a tunnel leading from the bottom of the crevasse to the rocky area.

The Robot

The Doctor, who is enraged at what Styre is doing, attacks the Sontaran, when he is discovered, but the Sontaran easily fends him off and leaves him for dead. The Doctor however, recovers. He then disables the robot, and meets Sarah, who has also recovered from her ordeal, and Harry, who also followed the tunnel at the bottom of the crevasse.

The Doctor then goads Styre to an unarmed combat. With the Styre distracted Sarah and Harry free the remaining astronauts. Harry then enters Styre's spaceship where he uses The Doctor's sonic screwdriver to remove a vital piece of equipment - a terrulian diode bypass transformer. Styre almost wins the fight with The Docor, but Vural attacks him, saving The Doctor at the cost of his own life.

However, being unfamiliar with Earth's gravity Styre has become weakened and so is forced to return to his ship to revitalise himself. Harry's tampering though causes the Sontaran to be drained of all his energy and so is destroyed. The Doctor then sends a message to the Sontaran Marshal, warning him that without Styre's report they cannot carry out their invasion plans. This is enough to ward off the invasion.

Having thwarted the Sontaran’s plans The Doctor, Sarah and Harry leave Earth via the transmat with the intention of returning to The Ark.