A new arrival at Rossini's International Circus, calling himself the Master, enlists the assistance of Rossini, firstly to steal a meteorite from the national space museum, then to hijack a radio telescope at a research establishment and kidnap a scientist.

After a rocky introduction to his new assistant, Jo Grant, the Doctor is alarmed when she informs him about the theft of the meteorite. On hearing about the sabotage at the research centre the Doctor decides it warrants some investigation. A Time Lord warns the Doctor that an old enemy, another renegade Time Lord known as the Master, has come to Earth. Apart from wanting to kill the Doctor, the Master is also likely to cause trouble. The theft of the meteorite, a container for part of the Nestene Consciousness and the sabotage at the centre suggest that the Master is helping the Nestene to invade Earth. Knowing that the Nestenes make use of plastic, the Brigadier has plastics factories investigated. Jo sneaks into one factory where she encounters the Master. He hypnotises her into killing the Doctor...and so the battle begins.

The Doctor and Jo have gone to Stangmoor prison to observe a demonstration of the Keller Machine, a new means of reforming hardened criminals. Apparently the machine extracts certain negative impulses from the brain to leave a rational, well balanced individual ready to take their place as a useful member of society. This, the 113th process, does not quite go according to plan when there appears to be a surge, though Kettering insists that it was only a minor malfunction and that the process has completed satisfactorily. Later on, when first a journalist and then Kettering himself die within the vicinity of the machine, and both havving died as a result of their own, deepest fears the Doctor realises that the machine is more than it appears to be.

The Brigadier is unable to help as UNIT is providing extra security for a world peace conference. In addition he has to arrange the transfer of a nerve gas missile. To add to his woes, important state documents have been stolen from the Chinese delegation and then a leading member of the delegation dies.

A large object is being tracked through space travelling towards Earth. As it enters the upper atmosphere, Chinn, a government minister orders a missile strike. The object disappears at the moment of strike: the Doctor calculates that it has shifted to another location and in fact may now be on Earth. The object is located near a major nuclear power complex. Bill Filer, an FBI agent sent to Britain to chase up a lead on the criminal known as the Master, finds himself excluded from investigating the object so he drives on ahead of the UNIT convoy and gets there first.

A signal from the object, identifying itself as Axos, turns out to be a call for help. The crew of Axos, the Axons need help to repair their ship. In return, they offer the human race the power of Axonite, a thinking molecule that can do just about anything. Chinn believes that as Axos is on British soil, Britain should have sole distrubtion rights of Axonite. The Axons wish that it be shared with the whole world. The Doctor believes that the scientists should study Axonite, but is his interest driven by altruism or by a desire to escape his exile from Earth? And just why have the Axons imprisoned Bill Filer...and the Master?

Worried about the disapperance of important, secret documents, the Time Lords enable the TARDIS so that the Doctor can perform a task for them. Emerging on the planet Uxarieus in the year 2472, Jo wants to return to Earth. For the Doctor, this is his first break away from Earth for a long time now. The urge to explore is great and so he sets off. Vehicle tracks suggest the planet is inhabited. Further along they spot some prefabricated structures. Before they can return to the TARDIS though a man with a gun escorts them to the nearby colony, a group of humans who have come to Uxarieus to make a better life for themselves. It has taken everything they had to get there; their ship is unlikely to be able to make the return trip; and the planet seems resistant to all attempts for subsistence. In addition, there are reports of giant lizards attacking some of the settlers.

When the colonist discover that they are not alone (barring the primitive natives of the planet) - a team from the Interplanetary Mining Corporation has arrived on the planet - they are faced with having to abandon the planet, their dream. The mining combine wants them off the planet as soon as possible and apparently will stop at nothing to persuade them to leave. Their only hope would be for an adjudicator to come to the planet and make a decision. Will the adjudicator rule in favour of the colonists or acknowledge the needs of Earth and side with the miners? Could the ancient ruins and the primitive natives hide a secret, something of interest to a third party?

Miss Hawthorne is concerned that the forces of darkness are gathering in Devil's End: Professor Horner's archaeological dig at a local barrow known as the Devil's Hump (to be televised); the new vicar - the rather sinister Mr Magister; mystery deaths and the dig being scheduled for Beltane. At midnight, Horner intends to break through the final part of the barrow to excavate Bronze age relics.

The televised dig has Jo, Yates and Benton among its viewers. The Doctor has little interest until he hears the name of the village. Something is bothering him and then when Miss Hawthorne gatecrashes the broadcast, the Doctor decides that he must go to Devil's End. He agrees that the dig should not go ahead. The Doctor and Jo attempt to get there before midnight hit a snag when the sign they follow takes them in the wrong direction (as if someone was trying to stop them). With Jo's help, a study of the local map guides them to Devil's End.They arrive just as Horner breaks through the last wall...and something is unleashed.