Journalist Sarah Jane Smith covertly gains access to a research center where top scientists are being held in protective custody while 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 neighboring castles. The Doctor helps Sir Edward of Wessex to repel one such attack, and 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.