Plot

Part 1

Something is amiss in the mining village of Killingworth in early 19th century England. Miners are being gassed in the bathhouse and turned into thugs and vandals, attacking men and machinery, seen as Luddites by other locals. The Sixth Doctor and Peri Brown witness this when they arrive in Killingworth looking for the cause of some sort of time distortion. They also notice one of the rampaging miners has a strange red mark on his neck. With his usual audacity, the Doctor foists himself upon the local landowner, Lord Ravensworth, who is concerned at the ferocity of the attacks, with the most passive of men suddenly turning violent and unpredictable.

The answer lies in the local bathhouse. The Master has turned up at this key point in human history. He forces his way into the presence of the old woman who runs the bathhouse, in reality another Time Lord, the Rani. She is a gifted chemist and is using the set-up of the bathhouse to anaesthetise the miners and distill from them the neuro-chemicals that enable sleep. This is what causes the red mark on the victims. These chemicals are synthesised for use back on Miasimia Goria, a planet she rules and which the Master has visited, where her other experiments have left the inhabitants without the ability to rest. He persuades her they need to deal with the Doctor together. He also steals some of the precious brain fluid she has collected to ensure her collaboration. It is a rocky partnership, full of half-truths and deceptions. The Master goes to deal with the Doctor, egging on local miners to attack his enemy and persuading some to throw the Doctor's TARDIS down a mine shaft.

The Doctor has meanwhile dressed as a miner and entered the bathhouse. He soon deduces the Rani's schemes. She traps him but he challenges her ethics. She reveals she has been coming to Earth for centuries to harvest her precious chemicals. The Master convinces the Rani to let him deal with the Doctor. He has the TARDIS pushed down a mine shaft. The angry Luddites put the Doctor in a cart to ensure that he follows it.

Part 2

The Doctor is saved by inventor George Stephenson. He and Peri return to Lord Ravensworth's, where Stephenson has planned a meeting of scientific and engineering geniuses in the village. The Doctor is worried about the wisdom of such a meeting in the current circumstances, but the Master is so desperate to see the event take place, he uses mind control of Stephenson's young aide, Luke Ward, telling him to kill anyone who tries to prevent it. The Master wants to use the finest brains of the Industrial Revolution to speed up Earth's development and then use the planet as a power base. He strikes a deal with the Rani; she may return to Earth at any time to harvest more brain fluid if she helps him achieve this.

While the villains are away, the Doctor returns to the bathhouse and dodges booby traps to enter the Rani's TARDIS. Her control room holds jars of preserved dinosaur embryos. She summons her ship to the old mine workings with a remote control device, with the Doctor still inside. He hides while his adversaries talk. The Rani has also set land mines in nearby Redfern Dell. When the coast is clear, the Doctor slips away to report back to Ravensworth, Stephenson and Luke, who is behaving strangely.

Peri is making herself useful by using her botanical knowledge to make a sleeping draught for the afflicted miners. Her quest for herbs leads her to Redfern Dell. The Doctor surprises the Master and the Rani, who are lurking at the edge of the dell. Soon after he does, he sees Luke steps on a mine and get turned into a tree. He takes the other two Time Lords prisoner with the Master's own Tissue Compression Eliminator. Peri takes charge of them but the Rani's deviousness outstrips the Master's and she is the one who enables them to escape. The Rani and the Master flee in her TARDIS, but the Doctor also has a trick or two: he has sabotaged the navigational system. The ship is out of control. In the destabilised condition, one of the jars holding a Tyrannosaurus Rex embryo falls on the floor and the creature, affected by the time spillage, starts to grow.

The Doctor and Peri make a swap with Ravensworth, who has retrieved the TARDIS. He gets the phial of brain fluid, which he is told to give to the affected miners. Before the eyes of an astonished scientist and his financier, the TARDIS departs...