In 1820's England, a group of miners finish a hard day's work. Jack Ward, Rudge and Green stop at a bath house run by an elderly crone. But as they strip, gas fills the room, knocking them out.
The Doctor and Peri are travelling to the opening of Kew Gardens when the TARDIS is drawn off course.
Two masked men return the miners to the bath house — with bright red marks on their neck. They begin fighting and race out into the street, where they attack passers-by.
The Doctor traces the time distortion through the countryside. He and Peri are watched by a nearby scarecrow.
Jack Ward and his fellows attack a cart, smashing a mining machine. The Doctor and Peri help the driver, who is concerned about what George Stephenson will think about the damage. The Doctor delightedly accompanies the driver to meet Stephenson.
The scarecrow maintains pursuit. As they drive into town, the crone also takes notice of them. The scarecrow discard his clothes to reveal the Master.
The Doctor and Peri reach the coal pit, where the Doctor tries to bluff his way past the guards. He finds out a meeting is soon to take place among some of the finest minds in England. Peri charms the guard into letting them in. They are placed in an office, guarded by a dog.
The dog get spooked and races out. The Master is at the gate. He destroys both the dog and a guard with his Tissue
Compression Eliminator. He then finds Jack Ward and his friends and tells them to attack the Doctor, explaining that the Doctor is an inventor who will deprive them of their jobs.
The men attack the Doctor, nearly pushing him into a pit. But he is saved by Lord Ravensworth, the pit owner. who frightens the men off with a gun. He has the Doctor taken back to his office and demands an explanation.
Ravensworth explains that there has been a recent explosion of violence. He is mystified — the men seem to suffer a complete change.
The old crone is attending to unconscious men, attaching sophisticated electronics to them. The Master breaks into the bath house and confronts her. She pulls off her disguise to reveal a younger Time Lady — the Rani. She knows the other Time Lords but is uninterested in their feud. The Master threatens her work and she has her men attack him. But she backs off when he destroys one with the TCE.
The Rani uses a green worm to take control of one of the unconscious men. The Master takes the parasites from her for his own use. The Rani explains that she is extracting the chemical that enables humans to sleep. She needs it to control the aliens on the planet she rules as her experiments deprived them of the ability to sleep, causing chaos.
Peri tries to get the Doctor to leave, but he wants to find out what's going on. He inspects the dead Rudge and finds the red marks.
The Rani uses a scanner to find the Doctor. The Master decides to take care of him, making sure the Rani stays behind. He takes her precious vial of brain fluid with him.
The Doctor talks to Jack Ward's son Luke. He realizes that his time tracer reacted near the bath house and that the men who attacked him were clean. He dresses up as a miner and spreads coal on his face.
The Master goes into the mine, where he is attacked by one of the Rani's men. Fearing the vial of brain fluid will be smashed in the struggle, however, she activates a control that destroys him. Jack and the other miners ask what happened and the Master tells them the miner was killed by the Doctor, using the TARDIS.
The Doctor joins two miners in the bath house. They are quickly knocked out by the gas. The Rani starts extracting the brain chemicals but recognizes the Doctor. She revives him and the Doctor quickly figures out what she is doing. She leaves him bound to a cart as he searches for the Master.
Peri watches her leave and breaks into the bath house. But she is unable to free the Doctor before the Master and the Rani
The Master explains his intention to change history. He then shows the Doctor an image of the Luddites about to throw the TARDIS into the depths of the mine. The Doctor pretends to think it's merely an illusion, convincing the Master to roll him outside to see it for himself.
As the miners toss the TARDIS into the pit, the Doctor kicks the TCE out of the Master's hand and has Peri push him away. But the cart rolls out of control right into the midst of the Luddites. They pull the stretcher up, place him on a mine cart and send him hurtling toward the edge of the pit…