Two schoolteachers, Ian Chesterton and Barbara Wright, become concerned by the unusual behavior of their fifteen-year-old student, Susan Foreman. When they follow her home, they meet her mysterious grandfather, the Doctor, and find themselves unwilling passengers on his time ship, the TARDIS...

The TARDIS takes its new passengers back to the dawn of history. There, the Doctor and his party are captured by a tribe who are trying to rediscover the secret of fire.

Old Mother frees the Doctor and the others on the condition that they will leave the tribe and not make fire. The group attempts to return to the TARDIS, but Za and Hur follow them.

Kal accuses Za of Old Mother's murder but the Doctor exposes him as the real killer. However, Za then insists the travellers be held prisoner until they show him how to make fire.

The TARDIS lands in a petrified forest on an alien planet. Determined to explore, the Doctor leads his companions into the metal city, where they discover the danger of what will become his deadliest enemies, the mutant Daleks.

The Doctor, Susan and Ian search the city for Barbara and learn that the area is contaminated with radiation.Then they encounter the Daleks.

Susan meets Alydon of the Thals and agrees to try and arrange a treaty between his people and the Daleks. But the Daleks only see it as an opportunity to wipe the Thals out.

The Doctor and his friends attempt to escape from the Dalek City. However, knowing that the Thals are walking into a trap, Ian decides to go and warn them.

The Doctor and his friends realise that their only chance of recovering the fluid link is to persuade the Thals to fight back against the Daleks.

While Ian, Barbara and their Thal friends try to find a way into the Dalek City through the mountains, the Doctor, Susan and Alydon attempt to make their own way in.

Ian, Barbara and the surviving members of their party try to find a way into the Dalek City whilst Alydon prepares the other Thals for an attack. But only the Doctor and Susan are aware of the full extent of the threat the Daleks pose.

In this completely TARDIS-based story, the crew find themselves and their ship acting very strangely indeed. Blame runs high for the Ship's unusual behavior, until the Doctor realizes the TARDIS itself may be trying to warn them...

The Doctor becomes increasingly suspicious of Ian and Barbara but when the occurrences aboard the TARDIS become even stranger he realises they are in even greater danger than they realised.