This very strong 4 part story is set on an apparently idyllic world where all is not as it seems.
The adventure brings the TARDIS crew into a moral stand against a system which exploits one group for the furtherment of the other. It is not the first or last time such issues arise in the long running series but the ideas are well enough written and presented here to make this a very successful and entertaining story overall.
It is not necessarily 100 percent perfect but the story is interesting and well scripted by Ian Stuart Black. There is action, intelligent themes, good dialogue and some fine acting. The leader of the Elders, Jano (Frederick Jaeger), is an especially good character. William Hartnell and Peter Purves deliver great performances as usual and the fact it is the last story for Peter Purves as Steven is very sad. He was one of the best companions in my opinion but at least he had a decent send-off.
I think this story perhaps is the first sign of producer Innes Lloyd and story editor Gerry Davis, who both took over their roles mid season, beginning to put their stamp on the show. They apparently wanted the show to be less whimsical and more 'gutsy' with more action and seriousness in the scientific themes. This ticks those boxes.
My Ratings: Episode 1 - 8/10, Episode 2 - 9.5/10, Episode 3 - 8.5/10, Episode 4 - 9.5/10