In the far future a group of humans is living an idyllic existence on a distant planet. Their colony is run like a gigantic holiday camp and nothing seems to trouble their carefree existence. When one of them claims that the colony is being invaded by hideous monsters, no one takes him seriously. But the Doctor's suspicions are immediately aroused. What is the terrible menace that lurks at the heart of this apparent paradise? Why are the colonists unaware of the danger that lies before their very eyes? And what is the Macra Terror?

Landing on a distant planet, the Doctor confidently announces to his companions that the TARDIS has brought them to an age of great advancement, peace and prosperity.

The Doctor's calculations seem to be confirmed when the travellers are greeted by Jano and the Elders who take them on a tour of their city - a haven of beauty, harmony and friendship, set in a wilderness inhabited by tribes of savages.

But the security of the city is founded on one deadly and appalling secret. Soon the Doctor and his friends discover that it is not only outside the city walls that savages dwell...

London, 1966 - when the TARDIS materialises outside the Post Office Tower the Doctor becomes aware of a powerful and evil force nearby, so he and the ever-curious Dodo set off to investigate.

Inside the tower they track down the source of power to Professor Brett and WOTAN, his revolutionary new computer.

Designed as a universal problem solver, WOTAN has suddenly begun to think for itself and has formulated a deadly plan. Using its phenomenal power it will programme humans to build mobile fighting computers, and with these indestructible war machines WOTAN will take over the world...