After a spate of murders, the villagers of Schtettel kill the depraved perpetrator, Count Mitterhouse. Fifteen years later the Circus of Nights appeared in the plague-ridden village and its performers include Mitterhouse's mistress, children and cousins. They have come to Schtettel to fulfil the Count's last words, an evil, vicious curse of death and destruction on those who participated in his impaling. The children of Schtettel become the targets for a brutal and devastating revenge as the Vampire Circus rehearses for its most deadly performance.
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.
