Captain Irongron (David Daker), a robber baron, and his aide Bloodaxe (John J. Carney), see what they believe to be a star falling in the nearby forest. When they go to investigate the following morning, they find a Sontaran officer, Linx (Kevin Lindsay), who claims the planet for the Sontaran empire. Linx wants Irongron's help to repair his ship and offers weapons in return. As the technology is primitive, Linx determines to gain scientific help from elsewhere. 

On Twentieth Century Earth, the Brigadier (Nicholas Courtney) has collected together all the top scientists into one top secret establishment. Several scientists have mysteriously vanished and UNIT are investigating. The Doctor meets journalist Sarah Jane Smith (Elisabeth Sladen), who posed as her aunt Lavinia Smith, the noted virologist, in order to get into the establishment. 

The missing scientists (George Ballantine, David Eynon, Eden Fox, Roger Marston, Paul Phillips, Frank Seton) have been kidnapped by Linx, who has hypnotised them and is using them to repair his ship. Irongron's behaviour has concerned his 'neighbours', Edward of Wessex (Alan Rowe), and his wife, Lady Eleanor (June Brown). Edward hopes to band the other local castles together against Irongron and has sent a message with his squire, Eric (Gordon Pitt). Eric is captured by Irongron and Linx hypnotises him to give Irongron Edward's plans. 

The Doctor sets up a rhondium sensor to detect delta wave particles and wake him up. That night, the machine operates and the short sighted Professor Rubeish (Donald Pelmear) vanishes from the security establishment. The Doctor uses another device to sweep the area and sees the ghost-like figure of Linx on the stairs. He hurries to follow the trail left by the Sontaran in the TARDIS. 

The TARDIS arrives in Irongron's time but, unknown to the Doctor, Sarah entered the TARDIS looking for Rubeish, and is now in the past with him. Lady Eleanor has arranged for Hal the Archer (Jeremy Bulloch) to shoot Irongron from his battlements but Sarah distracts his aim and both he and she are captured by Irongron. Linx interrogates Sarah as he recognises that she is wearing machine-woven clothes. He then shows Irongron his new weapon, a fighting robot (John Hughman, Dudley Long, Bill Monks). Sarah slips away. Hal is made to fight the robot but he escapes when the Doctor shoots the control unit from Irongron's hand with a crossbow. Sarah and Hal escape from the castle. 

The Doctor finds Linx's workshop and discovers that Rubeish has escaped hypnotism as he cannot see. Linx enters and the Doctor offers to help him but Linx will not listen and stuns the Doctor with his gun. With the Doctor immobilised, Linx explains that he was with the Fifth Sontaran Army Space Fleet and that he had been attacked by Rutan fighters while on a reconnaissance mission. Linx attaches the Doctor to a machine that delivers electric shocks if he tries to escape. Rubeish releases the Doctor when Linx has gone. 

Meanwhile Sarah has hatched a plan to kidnap the Doctor as she thinks he is behind the scientific knowledge being gained by Irongron. She and Hal lead a raid on the castle and the Doctor is captured. 

The Doctor agrees to help Edward defend his castle when Irongron's men attack. He and Sarah instruct Edward's men to construct dummies to place on the battlements and the Doctor makes some smoke bombs from saltpetre, sulphur and fat, together with some extras to provide a noxious smell. Irongron's attack fails and he and his men retreat to their castle. 

The Doctor proposes capturing Irongron's castle before they can attack again and gives Lady Eleanor a list of ingredients - ragwort, henbane, love in a mist, fennel and sesame - to make a sleeping potion. Dressed as friars, the Doctor and Sarah get past the sentry (Steve Brunswick) into Irongron's castle. While the Doctor helps Rubeish and the other scientists, Sarah heads for the kitchens where she is found by Meg (Sheila Fay), the cook, and made to work preparing food. Sarah pours the potion into the cooking pots when the opportunity arises. 

The Doctor manages to break Linx's hypnosis by flashing a penlight to polka rhythm in the scientists' eyes. Linx interrupts but the Doctor manages to keep him talking while Rubeish hits the Sontaran over his probic vent - a hole in the back of his neck - which stuns him. The Doctor ties him up and then dresses as the robot to delay Irongron. His ruse is discovered, however, and Irongron decides to use the Doctor for target practice with his new guns. Irongron goes looking for Linx and releases him. As the Doctor dodges the guns, Sarah comes to the rescue and the two of them return to Edward's castle to wait for the potion to take effect. 

Linx completes the repairs to his ship and starts the power build up to take off. The Doctor, Sarah and Hal return to the castle. The Doctor uses the osmic projector from the Sontaran ship to send the scientists back to the Twentieth Century. Hal goes to disarm Irongron's men but Irongron awakes and goes looking for Linx. Linx kills him before entering his ship. 

After waking Bloodaxe, who rouses the rest of his men to flee the castle, Hal returns to Linx's workshop where he fires an arrow into the Sontaran's probic vent, killing him. The creature activates his take off sequence, however, and so the Doctor, Sarah and Hal also flee the castle before it is destroyed as Linx's ship blasts off.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}