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.
| Actor | Role |
|---|---|
 William HartnellThe Doctor | The Doctor |
 Carole Ann FordSusan Foreman | Susan Foreman |
 Jacqueline HillBarbara Wright | Barbara Wright |
 William RussellIan Chesterton | Ian Chesterton |
| SStephen DartnellJohn | John |
 Ilona RodgersCarol | Carol |
| LLorne CossetteMaitland | Maitland |
 John BaileyCommander | Commander |
| MMartyn HuntleyFirst Human | First Human |
| GGiles PhibbsSecond Human | Second Human |
 Peter GlazeThird Sensorite / City Administrator | Third Sensorite / City Administrator |
| AArthur NewallFourth Sensorite | Fourth Sensorite |
| EEric FrancisFirst Elder | First Elder |
| BBartlett MullinsSecond Elder | Second Elder |
| AAnthony RogersSensorite | Sensorite |
| JJoseph GreigSecond Sensorite / Second Scientist / Warrior | Second Sensorite / Second Scientist / Warrior |
| KKen TyllssenFirst Sensorite / First Scientist | First Sensorite / First Scientist |
| GGerald MartinSensorite | Sensorite |