Following the encounter with the Aztecs,a now admittedly close knit TARDIS crew arrive on an Earth spaceship where it's two person crew,Carol and Maitland is revived from unconsciousness that they were put into by a seemingly hostile race the call the Sensorites. The ships mineralogist John has apparently gone mad. He was sent by this 28'th century Earth vessel to the sense sphere,home of the Sensorites to retrieve the valuable mineral molybdenum where an earlier scout team had failed earlier. However the Sensorites seem to have some mind control over the human crew. And Susan begins to develop an awakened telepathy which allows her to communicate with the very telepathic Sensorites. Barbara and Ian are able to strike the deal with the Sensorites,who are desperately trying to cure a deadly disease on their planet to help them in exchange for the release of the TARDIS,of which the Sensorites damaged the lock of.
On the sense sphere,the first elder Sensorite opens his heart to the friendly newcomers. Though admittedly many people blame the previous human encounter for the illness now afflicting them. That is,until Ian himself comes down with the illness. The Sensorite society is one of well ordered equality where all labors are allocated by a caste system based on ability. This prevents the first elder,in addition to the virtually identical physical appearance of each Sensorite,to realize the illness comes directly from his deceptive,power seeking second elder. He has poisoned the water aqueduct with night shade poisoning,using a previous human expedition as scapegoats. The Doctor of course discovers an antidote and is even able-with the help of Barbara,Ian,Carole and her newly cured fiancee John in uncovering the traitorous second elder before leaving the sense sphere to continue their voyages.
This serial is actually one of the best plotted and written of the first season of Doctor Who. The Sensorites are very interesting alien characters. Not able to physically tell each other apart,they've developed a society based on absolute trust based on their heightened senses. However this also leads to vulnerabilities. They are unable to see traitors in their own midst,for one. For another any loud noise creates great physical pain for them. So it's up to Susan,who as a character is fast developing her own way of thinking (much to the Doctor's chagrin),to try to communicate their good intentions to the Sensorites telepathically. It's an excellent story about a young woman's coming of age as well as knowing when to trust,and when not to. The blind trust of the decent but somewhat naive Sensorites isn't far removed from how real life human beings place too much faith in those higher in power than themselves. Overall a very telling and highly underrated early Doctor Who tale.