The Doctor and companions land on an island with a pyramid shaped tower. I saw that tower and thought Atlantis but the story had nothing to do with Atlantis, so much for that. In any case they immediately fall into intrigue and a planet full of mysterious peoples. The tower contained a remnant of the technological development of the people of that planet, it contained a machine that broadcast a signal which made everybody agreeable and peaceful. But some were not affected by the machine and there had been a war etc. The machine was operated by five keys, and the keys had been scattered for safekeeping. The Doctor and Companions then are sent on a quest to travel across the planet and recover all the keys.
As I said they come across many strange peoples. It is a very misleading story full of twists and turns.
The first place they visit is the City of Morphoton seemingly inhabited by a race of gentle people who have everything they need and willingly give all that's desired by the Doctor and Companions. However it's all a hypnotic illusion and they barely escape without being enslaved.
The story goes on like this -- there is an outpost in a jungle where a mad scientist has somehow unlocked the secret of amazingly fast plant growth, creating the jungle. Another place is in snowy mountains with a rogue who attempts to trick the Companions and kill them by stranding them in the blizzard. The final place is a city in which one of the Companions is accidentally implicated in murder.