With Jo held prisoner in his TARDIS, the Master forces the Doctor to take him to the underground city of the primitives. Any sign of treachery from the Doctor and Jo will die.
In the underground city, the Doctor and the Master confront the guardian of the Doomsday Weapon. The miners make their last move to force the colonists to leave the planet.
The Doctor tries to prevent Professor Horner from breaking into an ancient barrow near the village of Devil's End. Posing as the village vicar, the Master summons dark forces.
Devil's End is sealed off from the outside world by a heat barrier. The Doctor and Jo return to the barrow where they are confronted by the gargoyle, Bok.
The Doctor attempts to help the Brigadier penetrate the heat barrier, but is attacked by the Master's servant in a helicopter. The Master prepares to summon the Daemon Azal.
The Doctor is captured by the villagers who are under the Master's control. Jo goes to the church to prevent the Master from summoning Azal for the final time, but is too late.
UNIT have penetrated the heat barrier and launch an attack on the gargoyle, Bok. In the cavern, the Daemon Azal confronts the Doctor and the Master to decide the fate of mankind.
The Master, a renegade Time Lord and a rival of the Doctor's, arrives on Earth and paves the way for an invasion by the insidious Nestene Consciousness.
The Doctor follows the Master's trail to the circus. Meanwhile, the Master has used his hypnotic powers to take control of a plastics factory where he creates the deadly Autons.
The Doctor and the Brigadier trace the Master to the plastics factory but they are too late. The only clue to the Master's plans is a single plastic daffodil…
The Doctor and UNIT race to prevent the Master from broadcasting the deadly activation signal and opening the channel for the Nestene's arrival on Earth.
At Stangmoor Prison, the Doctor and Jo watch a demonstration of the Keller Machine which can extract evil impulses from criminals. But the experiment goes wrong and a man dies.
A full scale riot takes place at the Prison and Jo is taken hostage. Meanwhile, at the World Peace conference, the Chinese delegate's aid prepares to kill the American delegate.
The Master is behind the Keller Machine and uses it to recruit the rioting inmates at Stangmoor Prison. He plans to capture a missile and fire it at the World Peace Conference.
The mind parasite within the Keller Machine becomes too powerful for the Master to control. It learns how to move of its own accord and goes on the hunt for new minds.
The Master forms an uneasy alliance with the Doctor so that between them they can subdue the Keller Machine which is now out of control.
The Keller Machine breaks free of the Doctor's restraining device and the Doctor has no choice but to turn the parasite against the Master.
Axos lands on Earth. The Axons ask only to be allowed to repair their damaged ship and offer the gift of the miraculous Axonite in return. But the Axons aren't all that they seem.
The Master is in league with the Axons who intend to consume Earth. The Doctor subjects Axonite to tests and discovers the terrible truth.
The Axons reveal their true nature and attempt to obtain the secret of time travel. Meanwhile, the Master, forced to work for UNIT, concocts a plan to strike back at Axos.