Journalist Sarah Jane Smith covertly gains access to a research centre where top scientists are being held in protective custody whilst UNIT investigates the disappearance of a number of their colleagues. The missing scientists have been kidnapped by a Sontaran, Linx, and taken back to medieval England, where they are working under hypnosis to repair his crashed spaceship.

The Third Doctor follows in the TARDIS, and Sarah Jane stows away. In return for shelter, Linx has provided a robber baron called Irongron with anachronistically advanced weapons to use in attacks on neighbouring castles.

The Doctor helps Sir Edward of Wessex to repel one such attack, then he and Sarah Jane conspire to drug the food in Irongron's kitchens so that the weapons can be removed while the men are unconscious. Aided by one of the kidnapped scientists, Rubeish, he then sends the others back to the 20th Century using Linx's primitive time travel equipment.

Linx shoots Irongron down and gets ready to leave in his repaired ship. Hal, one of Sir Edward's archers, fires an arrow into the vulnerable probic vent at the back of his neck, killing him. The Doctor, Sarah Jane and Hal escape just before the ship explodes, destroying the castle.

 

The Fourth Doctor, Sarah Jane Smith and Harry Sullivan arrive on a desolate and apparently deserted Earth to discover that a group of shipwrecked astronauts from a human colony, GalSec, have been lured there by a fake distress call. One of their number, Roth, tells Sarah Jane of an alien conducting gruesome experiments on him and his crewmates. The alien turns out to be a Sontaran, Field-Major Styre, who is compiling a report on human physical and mental capabilities as a prelude to an invasion of Earth.

The Doctor challenges Styre to unarmed combat. The Sontaran agrees but is quickly weakened in Earth's unfamiliar gravity. Harry meanwhile enters Styre's ship and uses the Doctor's sonic screwdriver to remove the vital terullian diode bypass transformer, so that when the alien returns there to revitalise himself he is drained of all his energy and destroyed. The Doctor sends a message to the Sontaran fleet, warning them that without Styre's report they cannot invade.