Doctor Who, an eccentric inventor, lives with his granddaughters, Barbara and Susan Who, in a suburban house. Barbara is a teenager; Susan is a little girl. One evening, Barbara's boyfriend, Ian Chesterton, comes to call.
Doctor Who takes Ian and Susan into the garden to look at his new invention, TARDIS, a machine for travelling through time and space. It looks like a police box, and seems larger on the inside than on the outside. Barbara joins them in TARDIS. Embracing Ian, she pushes him on to a lever that sends the TARDIS to a distant time and place.
Finding themselves in a seemingly deserted petrified forest, the travellers spot a futuristic city in the distance. Doctor Who and Susan are keen to explore, but Ian wishes to return to earth. Doctor Who claims that he needs some mercury for a part called the fluid link. Without this, TARDIS cannot return to earth. They approach the city to look for mercury.
When they reach the city they are taken prisoner by its residents, the Daleks, alien creatures who live in metal casings to protect themselves from radiation. They confiscate the fluid link. The humans have been made weak from radiation poisoning, the result of the planet's toxic atmosphere. This was caused by a nuclear war many years ago between the Daleks and the Thals, a humanoid race who live deep in the forest.
The Daleks know that the Thals have a drug that cures radiation sickness. They let Susan out of the city, hoping that she will fetch the drug and lure the Thals to the city, where the Daleks plan to destroy them. Returning to TARDIS, Susan meets Alydon, leader of the Thals. He is surprised to hear that there are Daleks living in the city, and gives Susan a supply of the drug.
Susan returns to the city and the humans take the drug, while the Daleks spy on their cell. The Daleks decide to lure the Thals to the city where they intend to kill them. They make Susan write a letter to the Thals offering them food. The Thals, receiving the letter, trek towards the city with food baskets.
The humans overpower a Dalek guard. Ian pulls out the alien within. He climbs inside the metal casing and pretends to be a Dalek, escorting his human prisoners. The other Daleks realise that they are being deceived and chase the humans. Meanwhile, the Thals arrive at the city; Doctor Who reveals that they are walking into a trap; the Thals flee with the humans.
Doctor Who encourages the Thals to fight. They profess to be a peaceful people. Meanwhile, the Daleks, who have tested the drug, find that it does not work on their own kind. So they decide that they will explode a new, more powerful neutron bomb to destroy the Thals.
Doctor Who reveals that there was never anything wrong with the fluid link, and they will have to return to the city to retrieve it. He convinces the Thals that they can fight the Daleks; they plan to get into the city in two groups. One group, including Doctor Who and Susan, approaches the city walls; another group, including Barbara and Ian, approaches through a swamp.
At the gates of the city, Doctor Who and Susan are again taken prisoner by the Daleks; the Thals flee. Later they regroup and decide that they will return to the city and fight the Daleks.
The countdown to the detonation of the Dalek bomb begins. The remaining Thals break into the Dalek city, where they are pursued by Daleks. Thals and Daleks battle in the main control room of the Dalek city. Ian manages to stop the bomb countdown by tricking the Daleks into firing their weapons at their own control console. This also kills all the Daleks. Doctor Who recovers the fluid link.
The Thals perform a ceremony honouring the humans. The travellers leave in TARDIS, expecting to go home, but instead end up on a battlefield in Ancient Rome.
