Parts: 4
Writer: Robert Holmes
Airdates: 15 December 1973 - 5 January 1974
Production Code: UUU

Doctor: third
Companion: Sarah

Creature Feature: Sontarans
Special Guests: UNIT (the Brigadier)

Story: Late one night in England of the Middle Ages, an enormous and very bright star falls from the heavens. The residents of Lord Irongron's castle are startled, so the next morning, Irongron leads a hunting party to investigate. What he discovers is a spheroid spaceship, complete with armored occupant. The helmeted being shows off his weapons, and Irongron is suitably impressed. He agrees to give the being a space to repair its ship, in exchange for such weapons.

Meanwhile, in the present, the Doctor is concerned about a scientific conference that the Brigadier is overseeing - it seems to him that such a collection of great minds could prove irresistible to some nefarious power. He is proven correct when a machine built to detect the presence of delta particles goes off, just prior to the disappearance of one of the scientists. The Doctor takes the TARDIS to follow the trail, not realizing that he has accidentally brought a passenger with him - journalist Sarah Jane Smith.

Once he arrives in the Middle Ages, the Doctor locates the missing scientists and confronts Linx, a Sontaran warrior who is determined to get back to his battle. He has kidnapped the scientists to repair his ship, and is supplying Irongron with futuristic weapons to keep him pacified. Sarah, however, suspects that the Doctor is behind the kidnappings, and persuades Irongron's enemies to help her capture him. Unless the Doctor can convince Sarah of his intentions and stop Linx's selfish quest, the Sontaran may succeed in irrevocably changing the course of human history.