Irongron is bemoaning the fact that supplies are low: it has been sometime since his men have been foraging for supplies. Perhaps a visit to their neighbour, Sir Edward, will help replenish their food and drink? A falling star attracts Irongron's attention: come the dawn, he and his men ride out to it. The star opens and a strange creature emerges, claiming the planet for the greater glory of the Sontaran Empire. His ship is damaged and he needs the tools and the expertise to repair it. Unfortunately, Earth of the 12th/13th century has tools too primitive and no expertise to assist. Linx, in return for shelter and the chance to repair his ship offers weapons to Irongron, weapons that will help him defeat his enemies. He has other means by which he can obtain the expertise to help him repair his ship.

The Brigadier has placed a group of scientists in a secure location in the hope that this will put an end to the mysterious thefts of equipment and the disappearance of various scientists. The Doctor has agreed to help: he is intrigued by the appearance of prominent virologist Lavinia Smith - especially since the woman claiming to be her is too young. She reveals herself to be a journalist, Sarah Jane Smith - Lavinia is her aunt and she has taken her place. She too wants to learn more about these disappearances. When another scientist disappears the Doctor discovers that someone is operating some kind of time device from 12th/13th century England...

The Doctor and Sarah Jane have arrived in London: but an apparently deserted London; none of the phones appear to be working either; there are no buses; no moving cars; not even pedestrians. Except for one car that nearly runs them over. When they do find someone, he is robbing a store. Armed with a gun the robber tells the Doctor and Sarah Jane to keep away. They let him go - seconds later they hear the sound of a crash. Checking, the find the car, and the robber is lying next to it, dead.

Trying to take the stolen jewels to the police brings no joy either. The station is closed up. Sighting a van, the Doctor and Sarah Jane try to make contact with its occupants. They soon discover that something has caused London to be evacuated and that the only people in the centre are either looters or the military trying to enforce martial law and stop the looting. Trying to find a way to get around London, the Doctor makes use of a land rover - but when they are stopped by a patrol, the vehicle is filled with stolen goods. Arrested, the Doctor and Sarah Jane await processing...and soon they learn the nature of the threat that has invaded London.

En route to Florana, a planet with effervescent waters, some force affects the TARDIS, draining it of power. Even the back up systems are drained: so the Doctor has to use an oil lamp to provide some light and a crank handle to operate the doors enough to leave the TARDIS and find out more about the planet they have landed on. Expecting to arrive on the wonderful of Florana, Sarah is dressed for swimming and so the chill air of the world they have arrived on sends her back to the TARDIS for warmer clothing. She leaves the Doctor to have a look around, but when he returns to find him he has vanished. All Sarah can find is the abandoned oil lamp with blood on it. Strange figues are visible in the fog and she returns to the TARDIS for safety, however another creature has gotten inside forcing Sarah to flee. Lost in the fog, she finally comes to the edge of a huge city.

The Doctor is being escorted by his captors and when the time is right he breaks away. His escape nearly comes to a bad end when he encounters a trap, but he eludes that too. He meets the people who set the trap - humans from Earth. They are members of an expedition sent to this planet, Exxilon, to mine a mineral vital in the battle against a plague ravishing the Earth and its colonies. Like the Doctor, they too are stranded, some mysterious force has drained the power from their ship. With each passing day the plague will get worse. Their only hope is that a rescue ship will arrive soon, provided it too does not succumb to the power drain.

A group of miners are attacked by the spirit of Aggedor: it appears that he is angry about the mining that the Galactic Federation is carrying out on Peladon. Queen Thalira beleives that a demonstration of Federation mining technology might ease the fears of the miners. Instead, a further appearance by the Spirit of Aggedor and the death of another miner only serves to reinforce the miners fears.

The Doctor and Sarah Jane arrive on Peladon, some distance from the citadel. Making their way there the travellers encounter some guards and seek refuge, which turns out to be a temple for to Aggedor. Guards arrive and capture them, then bring them before Queen Thalira. The Doctor explains who he is, but is only the arrival of the Ambassador from Alpha Centauri, an old friend, that resolves the situation, for the time being.

Ortron does not trust the Doctor: after all it was his advice that led to Peladon joining the Federation and in effect has led to their current troubles; not to mention the fact that his arrival coincides with those same troubles. With the miners deciding to rise up against the rulers of Peladon it looks as if civil conflict is about to break out.

While enjoying rest and restoration at a meditation centre, Mike Yates, comes across a group of men having secret meetings. These men are attempting to access powers usually only the domain of masters and certainly not for novices. Sarah responds to Mike's invite: he hopes that she will be able to talk to the Brigadier as he thinks UNIT might be interested in what these men are up to. As Sarah and Mike watch the latest attempt produces a result: a giant spider appears, which attaches itself to the leader of the group, Lupton. Together, the giant spider and Lupton have ambitions for power. The spider has come to Earth in search of a blue crystal which can be a means to achieving such power.

The Doctor invites Professor Clegg to UNIT after seeing his stage act: but not because he wants him to put on a show for the troops; the Doctor has noticed that his act disguises his real abilities. The Doctor wants to help him so that Clegg can stop existing in the shadows and live a normal life. Setting up some equipment which will help the professor gain control over his abilities, the Doctor gives him a recently arrived parcel. The professor manages to determine what the parcel contains - a blue crystal. Originally from Metebelis 3, the Doctor had given it to Jo Grant. She has returned the crystal as tere is something disturbing about it. While the Doctor, the Brigadier and Benton discuss matters, Clegg picks up the crystal and stares into it, while still hooked up to the Doctor's equipment. A psychic storm rages through the laboratory and by the time the Doctor prises the crystal from the professor's hands it is too late - he is dead. The device has stored the images that had gone through Clegg's mind...images of giant spiders.