When Dr. Who shows his time machine TARDIS to the clumsy Ian, who is boyfriend of his granddaughter Barbara, he accidentally transport them and Dr. Who's granddaughter Susan to somewhere in space and time. They explore the spot and see a city; Dr. Who fakes a leak in the fluid and they go to the city to seek mercury to refill the component. They are captured by the Daleks and soon they learn that a war between Daleks and Thals has destroyed the planet. Further they are exposed to radiation and only the Thals have the antidote. The Daleks send Susan to find the cure and she meets the Thal Alydon that has the antidote and wants to negotiate with the Daleks to exchange for food. But the cruel Daleks want to destroy the Thals to rule the world.