Scientists go missing and the Doctor learns that the kidnapper is an alien from the past.
In the Middle Ages, the bandit Irongron and his aide Bloodaxe together with their rabble of criminals find the crashed spaceship of a Sontaran warrior named Linx. The alien claims Earth for his empire then sets about repairing his ship, offering Irongron “magic weapons” that will make him a king in return for shelter. They strike a bargain, though Irongron remains suspicious.
The Third Doctor and Brigadier Lethbridge-Stewart are investigating the disappearance of several scientists from a top secret scientific research complex. They do not know Linx has used an osmic projector to send himself forward seven hundred years and has kidnapped the scientists then hypnotized them into making repairs on his ship. The projector only lets him appear in another time for a brief period. While the Doctor investigates he meets an eccentric scientist called Rubeish and a young journalist called Sarah Jane Smith, who has infiltrated the complex by masquerading as her aunt. Later that evening Rubeish disappears and the Doctor uses the data he has gathered to pilot the TARDIS back to the Middle Ages, not realizing new companion Sarah has stowed away on board.
Irongron is a robber baron who has stolen his castle from an absent nobleman, and relations with his neighbors are appalling. Indeed, the mild Lord Edward of Wessex has been provoked into building an alliance against him and, when this is slow in developing, sends his archer Hal on an unsuccessful mission to kill Irongron. When Sarah follows the Doctor to Irongron’s castle, she is seized by one of his guards, while the Doctor witnesses Linx removing his helmet.