In the Middle Ages, the bandit Irongron (David Daker), his dim companion Bloodaxe (John J. Carney), and their troop find the crashed spaceship of a Sontaran warrior Linx (Kevin Lindsay). This is the first appearance of the popular Sontarans in the Who canon. Linx claims the Earth as his, then begins repairing his ship, giving the humans “magic weapons” for their help.
The Doctor and Brigadier Lethbridge-Stewart investigate the disappearance of several scientists from a top-secret research complex. They do not know, at first, that Linx has used his Osmic Projector to visit the 20th Century for a short period, kidnapped the scientists, hypnotized them and forced them to repair his ship’s drive. The Doctor meets an eccentric scientist called Professor Rubeish (Donald Pelmear) and a young journalist called Sarah Jane Smith (Elisabeth Sladen), who has basically snuck into the complex under false pretences. When Professor Rubeish disappears, the Doctor uses the fact to bring the TARDIS back to the Middle Ages. Of course, Sarah has stowed away.
Irongron has stolen his castle from an absent nobleman and does not play nicely with his neighbors. Lord Edward of Wessex (Alan Rowe) is building an army to attack him, but impatiently sends his archer Hal (Jeremy Bullock) to kill him. This fails because Sarah stumbles into the scene. She is arrested and brought before Irongron. Linx gives him a robot knight to kill Hal. The Doctor shoots the robot control box with a crossbow. In the confusion, Hal and Sarah escape to Wessex Castle.
The Doctor now knows that Sarah is there, and she thinks that he is in cahoots with Irongron. The next morning, Irongron assaults Wessex Castle with rifles, but the Doctor repels the attack. Linx and Irongron are on the outs now, particularly since Irongron has seen Linx’s face and knows he is not human.
The Doctor leads an attack on Irongron’s castle and he and Sarah disguise themselves as friars. He contacts Rubeish and finds that most of the scientists are exhausted. The hypnotism has not worked well with Rubeish because he was taken without his glasses, and he is able to help the Doctor. In fact, when Linx is about to overpower the Doctor, Rubeish strikes Linx on the vulnerable Promic Vent on the back of his neck. Rubeish and the Doctor use the Osmic Projector to send the scientists home, Sarah penetrates the castle kitchen and drugs the food.
Linx has completed his ship-repairs and wrestles with the Doctor. Half-drugged and all crazy, Irongron appears to confront Linx and the Sontaran kills him. But Link is wounded, and his ship takes off with him unconscious inside, causing an explosion that takes down the castle. Bloodaxe and his men flee, and the castle explodes just as the Doctor and Sarah take off in the TARDIS.
Wessex Castle and Irongron’s Castle were both played by Peckforton Castle in Cheshire. Jeremy Bulloch (the archer Hal) later played Boba Fett in the Star Wars Films. The name of the Doctor’s home planet, Gallifrey, was first mentioned here. The story was considered an enjoyable romp. Linx--Commander of the Fifth Sontaran Army Space Fleet--was praised, and the Sontarans appeared frequently thereafter. They were such popular villains that some appeared later as much-loved comically bloodthirsty good guys. Sarah Jane Smith was liked from the start by Jon Pertwee and the Doctor Who directors. Someone pointed out that, in their casting tryout, most were either frightened or brave, but she was both. But she still had to wade through the Women’s Lib jokes for a while.