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.
William Hartnell
as The Doctor
Carole Ann Ford
as Susan Foreman
Jacqueline Hill
as Barbara Wright
William Russell
as Ian Chesterton
Stephen Dartnell
as John
Ilona Rodgers
as Carol
Lorne Cossette
as Maitland
John Bailey
as Commander
Martyn Huntley
as First Human
Giles Phibbs
as Second Human
Ken Tyllsen
as First Sensorite / First Scientist
Joe Greig
as Second Sensorite / Second Scientist / Warrior
