The TARDIS travels far into the future and lands aboard a human spaceship in orbit about the Sense Sphere, planet of the Sensorites. The spaceship crew are held captive by the Sensorites, who have some sort of fluence over them; although the crew fear the Sensorites, the Sensorites haven't done anything threatening. The time travellers are also held captive because the Sensorites have removed the lock to the TARDIS.
The Sense Sphere is rich in Molybdenum, which the humans have discovered and want. It transpires that the Sensorites want to keep that fact hidden, especially because humans have been to the Sense Sphere before and that left the Sensorites suffering from a deadly illness.
Susan and the Sensorites develop contact through telepathy. An agreement is made. Barbara remains on the spaceship (a handy plot device really, as Jacqueline Hill was due for a holiday!), whilst the others go to the planet.
A power struggle develops amongst the Sensorites. The City Administrator fears the humans and wants to use a disintegrator gun to destroy "the creatures" . When Ian develops the same illness, the Doctor must hurry to find a cure. It turns out that some of the earlier human visitors are still there and have been poisoning the city water supply with Deadly Nightshade. They are defeated, the problem solved and the humans are allowed to leave upon promising to keep the secret of the Sense Sphere's Molybdenum.
This isn't an exciting story but it is OK . I like how the City Administrator is repulsed by the way humans look (a classic sci-fi alien response now), but furthermore wonders how the Sensesorites can be able to tell the difference between humans when "those creatures" wear no sashes or badges of office. The fun thing is the Sensorites all look like the two pictured above.
The story covers some of the things which could have been mentioned in the Aztecs about Cortes: explorers discovering gold amongst tribes and how some tribespeople died from lack of immunity to disease. In many ways, it is still with us, as multinationals lay waste to the Amazon after resources and meet resistance from the indigenous people. The interesting feature of the story is that the Sensorites only do what they do out of shear fear and distrust of the human intentions; they aren't malevolent or evil like the Daleks. It has a moral element.
Carole Ann has one of her best parts in this story. Susan stands up to Grandfather as a young woman, with her own mind, in this story. ****** The Sensorites