Something is amiss in the mining village of Killingworth, in 19th-century England. Miners are being gassed in the wash-house and transformed into thugs and vandals, attacking men and machinery, being perceived as Luddites by other locals (although, in reality, the Luddites targeted the textile industry and the new mass-production looms that threatened the usually skilled labor of the Luddite core group). The Sixth Doctor and Peri Brown witness the phenomenon when they arrive in Killingworth looking for the cause of some sort of time distortion, and he also notices 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 by the ferocity of the local Luddite attacks, with the most passive of men suddenly turning violent and unpredictable.
The answer lies in the local wash-house. The Master has turned up at this key point in human history and forces his way into the presence of the old woman who runs the wash-house: in reality another Time Lord known as the Rani. She is a gifted chemist and is using the set-up of the wash-house to anaesthetise the miners and distill from them the neuro-chemicals that enable sleep. This is what accounts for the red mark on the victims. These chemicals are then synthesised for use back on Miasimia Goria, a planet she rules and which the Master visited, where her other experiments have left the inhabitants without the ability to rest. He persuades her that they need to deal with the Doctor together, but also steals some of the precious brain fluid she collected to ensure her collaboration. It is a rocky partnership, full of half-truths and deceptions. The Master heads off to deal with the Doctor, egging on local miners to attack his enemy and persuading some of them to throw the Doctor's TARDIS down a mine shaft.
The Doctor has meanwhile dressed as a miner and infiltrated the bath-house, where he soon deduces the Rani's schemes. She entraps him but he still challenges her ethics, prompting her to reveal she has been coming to Earth for centuries to harvest her precious chemicals. After a series of narrow squeaks – during which the Doctor himself almost follows the TARDIS down the mineshaft – the Doctor and Peri escape their captors and return to Lord Ravensworth's where they meet the inventor George Stephenson. 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 elsewhere the Master is so desperate to see the event take place he uses mind-control over 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 help speed up Earth's development and then use the planet as a powerbase. He strikes a deal with the Rani that 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 wash-house and dodges the booby traps to find a way into the Rani's TARDIS. Her control room contains jars of preserved dinosaur embryos. She summons her ship to the old mine workings using a remote control device, with the Doctor still inside. He hides while his adversaries converse, with the Rani confessing to have also laid landmines in nearby Redfern Dell; and when the coast is clear the Doctor slips away to report back to Ravensworth, Stephenson and Luke, whom he sees is behaving strangely.
To make herself useful Peri is using her botanical knowledge to make a sleeping draught for the afflicted miners, but her quest for herbs leads her to Redfern Dell. The Doctor gets there in time to save her, but not before Luke accidentally steps on a mine and is turned into a tree. The Doctor then surprises the Master and the Rani, who are lurking at the edge of the Dell, and takes them prisoner with the Master's own Tissue Compression Eliminator. Peri is given 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 has also developed a trick or two: he has sabotaged the navigational system and the ship starts heading out of control. In the destabilised condition, one of the jars containing an embryo Tyrannosaurus rex falls to the floor and the creature begins to grow, affected by the time spillage.
The Doctor and Peri make an exchange with Ravensworth, who has retrieved the TARDIS and accepts the phial of brain fluid, which he is told to return to the affected miners. Before the eyes of an astonished scientist and his financier, the TARDIS departs…
Doctor Colin Baker (Sixth Doctor)
Companion Nicola Bryant (Peri Brown)
Anthony Ainley — The Master
Kate O'Mara — The Rani
Terence Alexander — Lord Ravensworth
Gawn Grainger — George Stephenson
Peter Childs — Jack Ward
Gary Cady — Luke Ward
William Ilkley — Tim Bass
Hus Levent — Edwin Green
Kevin White — Sam Rudge
Martyn Whitby — Drayman
Sarah James, Cordelia Ditton — Women
Richard Steele — Guard
Writer Pip and Jane Baker
Director Sarah Hellings
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6X
Series Season 22
Length 2 episodes, 45 minutes each
Originally broadcast February 2–February 9, 1985