The Doctor, Ian, Barbara and Susan arrive in the TARDIS on board a spaceship. Their initial concern is for the ship's human crew, who are suffering from telepathic interference from the Sensorites, but Susan communicates with the Sensorites and finds the aliens fear an attack by the humans and are just defending themselves. Travelling to the Sense Sphere (the Sensorites' planet) the Doctor seeks to cure an illness to which the Sensorites and Ian have succumbed, but finds it has been caused by deliberate poisoning. The political manoeuvring of the Sensorite City Administrator poses another threat to the TARDIS crew as he seeks to discredit and implicate them.
The occupants of the TARDIS are a bit puzzled when it stops but keeps moving at the same time! Maybe they are in something, or on something? Well that's what it turns out to be - they are on a ship that seems to be on auto-pilot with two dead crewmen still at their posts. Except, they are not dead - just in stasis and when they awaken they warn the "Doctor" (William Hartnell) et al to get going before they face the monsters who are terrorising them. Too late. The aforementioned creatures - the "Sensorites", have pinched the lock to their time machine and left them stranded. Thing is, are these creatures really so nasty? They don't ever kill so the "Doctor" and "Barbara" (Jacqueline Hill) decide that trying to communicate might be better good idea - and that leads them to their home planet where the race are slowly dying out and attempts to help are being thwarted by more militaristic elements amongst the administration who want to see the back of these meddling aliens. When "Ian" (William Chesterton) comes down with the same sickness, it's soon a race against time.... This series is far too long, with six episodes taking a meandering route through a story that's a bit thin, very dialogue heavy, and just a bit dull.