마을 사람들 때문에 죽어간 뱀파이어의 저주에서 시작된다. “너희들 누구도 살아남지 못할 것이다. 마을도 사라질 것이다. 너희 아이들은 나의 생명을 되살리기 위해 죽어갈 것이다.” 15년 뒤 마을에는 ‘밤의 서커스’를 펼치는 무리가 찾아와 공연을 한다. 그리고 저주가 이루어진다.
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.
