The Sensorites

The Sensorites was the final science fiction adventure in season one. The TARDIS crew have landed on a spaceship which is being held prisoner by the Sensorites, a species which communicate through the power of thought. They negotiate with the Sensorites and are allowed down to the surface where they help the species fight a poisoned water source. To provide obstacles, one parliamentary Sensorite doesn't trust the newcomers and wants to seize power.

It's quite a strong story and the tension builds up nicely in the first half, with a battle of wits between the humans onboard and the Sensorite captors. I think that in the second half, the story starts to go downhill a little, but by the end, it perks back up into quite a neat little adventure.