A very confusing and intricately plotted series which ends up in a very interesting place. The Doctor and Companions find themselves on a spaceship where the inhabitants appear to be dead. They're about to say "whatever" and leave when it appears maybe they aren't dead. They're able to resuscitate the inhabitants and embark on this strange journey.
The inhabitants have been mind-controlled by a race of telepathic people who are afraid these people will usher in a wave of invaders who will pillage their planet. Rather than allow the human spaceship occupants to return home and bring back reinforcements, they are enslaved in a kind of hibernation.
The Doctor and Companions become ensnared, and one of the captors steals part of the TARDIS. So, uh, if the TARDIS is nigh on impregnable then how could part of it be stolen. Oh, yeah, plot device.
They eventually end up on the planet of the Sensorites, their captors, and the planet is the Sense-Sphere. They learn the Sensorites are being poisoned and it's a great big mystery by whom or how. The reason is discovered and is a very surprising (to me) reason.