Un demoníaco seductor muere en manos de los habitantes de un pueblo. Esto no será más que el inicio de sus problemas. Una epidemia de peste que arrasa toda la región será la causa de la hostilidad de los pueblos vecinos contra ellos. Es entonces cuando llega a esta población un terrorífico circo con un espectáculo monstruoso. Una mujer pantera, dos ninfas asesinas y un joven vampiro ávido de sangre vengarán la muerte del seductor endemoniado matando uno a uno los culpables.
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.
