England, the 12th century. Robber baron Irongron is recruited by Linx, a Sontaran warrior whose starship has been forced down on Earth. Linx is forced to use a time-travel device to steal resources and scientists from the future to repair his ship, but in doing so involves UNIT and the Doctor...

While the Doctor carries out an investigation into ESP, Mike Yates asks Sarah to look into some strange goings on at a meditation centre he's attending.

K'Anpo persuades the Doctor he must face his fear and return the crystal to the Great One.But first they need to get past Barnes and his men.

Lupton forms an alliance with the spider to steal the Doctor's blue crystal, while Sarah and Mike attempt to alert people to what they've seen.

Sarah is captured by the spiders' guards while the Doctor tries to help Arak and the others strike back against the spiders.

The Doctor escapes from the spiders' larder only to encounter the Great One, while Sarah makes a deal with the Queen Spider.

The Doctor tracks the missing scientists to Irongron's castle, where Linx is using them to repair his spaceship and construct advanced weapons for Irongron.

The Doctor escapes the castle with Sarah and Hal and must persuade Sir Edward to trust him before Irongron launches an attack on them.

The Doctor attempts to return the scientists to their own time, before Linx tries to use his spaceship and destroys the entire castle.

The Doctor and Sarah return to London in the present day only to find the city strangely deserted, while nearby UNIT are dealing with a series of monster appearances.

The barren planet Exxilon, some time in the future. The TARDIS is forced down by a total power failure and the Doctor and Sarah meet a stranded Earth crew who came here to mine for priceless medical supplies. But what is causing the power failures? And why have the Daleks come here too?

The Doctor and Sarah head to the meditation centre to try and get the crystal back while Lupton prepares to return it to Metebelis Three. But all their plans are disrupted when it is stolen by Tommy.

The Doctor and the MSC crew join forces with the Daleks to mine the parrinium, only to come under attack from the Exxilons.

The Doctor and Sarah meet up with Bellal, who helps them escape from the Daleks and explains the origin of the City.

The Doctor and Bellal continue to penetrate the City's defences while Sarah and Jill try to stop the Daleks leaving with the parrinium.

The Doctor attempts to solve the mystery of the monster invasion. His first move - the capture of a dinosaur.

Yates sabotages the Doctor's equipment to stop him finding the location of Whitaker's base but then the tyrannosaur escapes, placing Sarah in danger.

Sarah attempts to find a way off the spaceship while the Doctor tries to track down the source of the time displacements.

The Doctor has been framed as the monster maker by Finch and is on the run from the army, while Sarah attempts to alert UNIT to what Grover is up to.

The Doctor is rescued from Finch by the Brigadier and Benton and tries to persuade them to stop Grover before millions of people are erased from history.

The Doctor turns the Aggedor spirit against the Ice Warriors to aid the Peladonians in their battle but Eckersley attacks him with the security system.

Sarah and Alpha Centauri decide to try and alert the Federation to what the Ice Warriors are up to but first they have to escape from the throne room.

The Doctor is rescued by Gebek, prompting him to try and help the miners, but his advice results in Ettis launching another attack on the armoury.

The Doctor and Sarah pacify Aggedor and the Doctor encourages the Queen to make peace with the miners, but Ortron soon has him under arrest again.

The Ice Warriors take control of the planet and force the miners to return to work but Ettis takes the opportunity to try and destroy the citadel.