Weary miners come off shift in Killingworth, a mining village in early 19th-century England. Jack Ward, Rudge, and Green go to a bath house owned by an elderly crone, but as they start to strip down the room fills with gas and knocks them out. Two burly, silent men in gas masks carry them through a secret panel into a separate room, and when their work has finished, Ward, Rudge and Green are awake and violently hyperactive. The three miners dash from the bath house and attack an old man and child in the street outside before rushing off...

The TARDIS is deliberately drawn off course and materializes on a slag heap near Killingworth; the Doctor had been aiming for Kew Gardens, but now must investigate the alien influence responsible for bringing him here. He and Peri set off to investigate, taking a sensor to search for temporal distortion. On a nearby road, they witness Ward, Rudge and Green attacking a drayman and smashing his cartful of machinery; the drayman knocks Ward down with a shovel and is beaten by Rudge and Green, who rush off without waiting for Ward to recover. The Doctor helps Ward and notices a red mark on the side of his neck before Ward flees.

The recovering drayman mentions that he was taking the machinery to George Stephenson, and the Doctor recognizes the name; Stephenson is one of the architects of the Industrial Revolution, and his presence near an alien time disruption may not be a coincidence. He and Peri accompany the drayman into the village, unaware that a scarecrow from a nearby field has climbed down from its stake and is following them. As they pass the bath house, the Doctor's time detector beeps briefly, and the old crone hears the sound. Worried, she bars the door behind her after the next shift of miners enters her bath house; the Master, disposing of his scarecrow disguise nearby, is insulted by the primitiveness of the barrier, but he's got other things to do before dealing with her...

The Doctor and Peri attempt to enter the pit but are barred by a guard, who refuses to let them enter since their names aren't on the invitation list. The Doctor bluffs his way in and notices that the names on the list are all famous historical figures, the architects of the Industrial Revolution. The guard takes them to Lord Ravensworth's office to await questioning by the mine owner, but the dog he leaves to guard them overhears something else and darts back to the pit entrance. The Master, who was trying to break in, is forced to kill both the dog and a guard who witnessed the unusual murder. He then notices Ward, Rudge, and Green passing by, and tells them that the Doctor is an inventor who's come to automate the mine and rob them of their jobs.

The Doctor and Peri have taken the opportunity to explore the pit head, but they are attacked by Ward, Rudge and Green and the Doctor is nearly pushed into the open pit. Rudge falls in to his death, but as Green and Ward try to knock the Doctor in after him, Lord Ravensworth arrives and drives them off. Ravensworth has the Doctor taken back to his office, where he admits that the Luddite attacks in this village have been more violent than anywhere else in the country -- and most of the attackers have been friendly, peaceful men who have suddenly become violent for no reason. Also, two miners, Josh and Tom, have disappeared, abandoning their wives and children.

The Master breaks into the bath house, and sees the old crone draining fluid from the miner's brains with an electronic distillation unit. The "crone" is in fact a renegade Time Lady, the Rani, in disguise, and the Master threatens to expose her setup if she doesn't co-operate. He steals a tin full of chemically impregnated maggots, which the Rani has used to turn two miners (Josh and Tom) into her servants. The Master also steals the brain fluid she's collected so far, and forces her to explain that she needs it for her aliens on Miasimia Goria; one of her experiments failed and her aliens became overly violent, and the Rani has been forced to come to Earth to harvest replacement neurochemicals. The Master points out that the Doctor is bound to discover and put an end to her work, and she agrees to help him destroy the Doctor; but he insists upon doing it his way, and keeps the brain fluid to ensure her co-operation.

The Doctor questions Ward's son Luke, and learns that Ward went to the bath house after finishing at the pit. The Doctor borrows a miner's outfit, spreads coal dust over his face and heads off to the bath house to investigate. Meanwhile, the Master goes to the old mine workings, trying to contact the hyperactive miners, but the Rani has laid a trap for him; somehow she is able to control the mines of the afflicted miners, and she sends Green to kill the Master. The struggle is fiercer than she expected, however, and fearing that the vial of brain fluid will be smashed, she kills Green instead by remote control. The Master manages to convince the other miners that Green's death was caused by the Doctor's TARDIS, and tells them to find it and throw it down the pit.

The Doctor enters the bath house with a group of miners coming off-shift, but he is overwhelmed by the gas fumes before he can find and block their source. The Rani straps him down in her laboratory, and when he recovers he finally realizes what's going on (although he's surprised to learn that the Master is also present). He soon deduces that the Rani is extracting the chemical which enables the human brain to sleep, but is unable to convince her that her actions are unethical; why should she care about lower life forms which interfere with her experiments? She's been coming to Earth for centuries, using the violence of its history to mask her own activities, and sees no reason to stop. She sets off to find the Master, ordering her chemically-hypnotised guards to kill the other miners if the Doctor tries to escape.

At the last moment, the Doctor is rescued by a stranger who seems more interested in the titanium-alloy straps restraining the Doctor than his actual predicament; the Doctor easily deduces that his rescuer is none other than George Stephenson. Stephenson leads the Doctor and Peri back to his workshop while the afflicted miners struggle with the pit guards. The Doctor advises Stephenson to call off his intended meeting of geniuses until it's safe, and Stephenson reluctantly acquiesces and sends Luke to take the message to Ravensworth. However, the Master runs into him on the way, and upon learning that Luke works for Stephenson, the Master uses one of the Rani's chemically-impregnated parasites to bring Luke under his control. If anyone tries to stop the meeting, Luke must kill them.

The Doctor and Peri leave Stephenson's workshop to investigate back at the bath house, but on the way they are attacked by Ward and another afflicted miner. The Doctor is nearly killed but escapes, and Ward and the other miner are taken prisoner. Meanwhile, Luke tells Stephenson that he delivered the message to Ravensworth, and that Stephenson is to remain in his workshop where it's safe. Luke then leaves the workshop and runs into Ravensworth, who's on his way to speak with Stephenson and advise him to call off the meeting. Luke manages to convince Ravensworth that Stephenson believes there's no danger -- fortunately for Ravensworth, since Luke was prepared to kill if he wasn't believed.

The Rani has nearly decided to cut her losses and go, but the Master convinces her that by harnessing the power of the geniuses attending Stephenson's meeting he can transform the Earth into the power base of his empire -- and if the Rani helps him, he will allow her to return to Earth openly and harvest as many humans as she likes. She's intrigued, and finally accepts his offer; but first they must deal with the Doctor. Her work here now finished, she casually kills her two slaves before she and the Master leave the bath house.

The Doctor and Peri break into the Rani's lab in the bath house, to find it empty apart from a room-divider with a volcanic landscape painted upon it. The Doctor, suspicious, ties a string to the divider and tugs, and the volcano erupts, filling the room with mustard gas. The Doctor and Peri survive by fitting themselves with the Rani's dead slaves' gas masks, and pull aside the room-divider to find that the Rani's TARDIS was powering the trap. They enter to find the control room full of preserved Tyrannosaur embryos, but before they can investigate further the console activates by itself, and the Doctor sends Peri out and remains in the Rani's TARDIS as it dematerializes.

The Rani summons her TARDIS to the old mine workings with a Stattenheim remote control, and the Doctor hides as she and the Master enter. She's disappointed to learn that her trap didn't work, and prepares to set a second trap by laying out specially programmed chemical land mines in Redfern Dell. The Master suggests luring Stephenson into the trap as well. As they leave, the Doctor emerges from hiding and operates the Rani's control console. He then departs, but while leaving the old mine workings he bumps into a loose support and nearly brings the roof down on himself.

Following the Master's orders, Luke tells Stephenson that Faraday was attacked by Luddites and is hiding out in Redfern Dell. Stephenson sends Luke to fetch help from Ravensworth, but when Luke arrives in the office he claims that he hasn't seen Stephenson for some time. The Doctor has returned to examine the afflicted prisoners, but he's suspicious of Luke's distant attitude and sets off to find Stephenson. Stephenson is preparing to set off for Redfern Dell, but the Doctor tells him to stay where it's safe -- and not to trust Luke. Meanwhile, Peri, looking for some way to help, offers to prepare a sleeping draught for the afflicted miners. Ravensworth identifies a local herb which might help, and Luke offers to lead Peri to collect herbs in Redfern Dell...

The Master and the Rani set their trap and wait for the Doctor to stumble into it; the Rani has by now deduced that the Master deliberately drew the Doctor's TARDIS here, seeking revenge and a means to provoke the Rani into co-operating with him. The Doctor, however, skirts around the Dell and surprises the Master and the Rani, taking them prisoner with the Master's own TCE. Before he can do anything, however, Luke wanders into the Dell ahead of Peri, steps on a mine and is transformed into a tree. The Doctor is infuriated, both by the cruel corruption and slaughter of an innocent bystander, and by the Rani's casual attitude towards the death; in fact, she claims that in a way, Luke is better off, as a tree lives four times longer than a human being. As the Doctor starts to lead them back to the Rani's TARDIS, Peri, unaware of Luke's fate, enters the Dell. A branch from the tree that was Luke swoops down and restrains her before she can step on a mine, and the Doctor forces the Rani to enter the Dell and guide Peri to safety.

As the Doctor leads his prisoners away, a party of afflicted miners approaches, heading directly towards the Dell. Peri saves the Doctor his moral decision by taking the TCE and forcing the Master and the Rani on, while the Doctor pauses only to whisper a warning to Peri and hurries off, bumping into the Master as he goes. His attempt to stop the miners fails, however, as they simply capture him, truss him up on a pole and carry him through the woods, ignoring his shouted warnings. The two miners carrying him step on mines and are transformed into trees themselves. The other miners flee in terror, and the Doctor manages to slip free of his bonds and uses the pole as a minesweeper to navigate his way out of the Dell.

As Peri guards the Master and the Rani in the old mine workings, the Master tries to hypnotise her, but thanks to the Doctor's warning she's prepared for this and ignores him. The Rani suddenly seems to suffer a seizure, and Peri allows her to take a capsule to ease her breathing -- but the Rani snaps the capsule under Peri's nose, instantly knocking her unconscious. The Rani decides to cut her losses and escape, but the Master orders her to remain, certain that the Doctor will return for Peri. The Doctor returns and is furious to realize that the Master has remained behind. He shows himself briefly, provoking the Master into firing at him -- and the Master inadvertently disintegrates a tunnel support, causing the loose support the Doctor noticed earlier to give way. The Doctor and the recovering Peri flee as the mine collapses around them, and the Master and the Rani are forced to retreat to the Rani's TARDIS and dematerialize. They realize too late that the Doctor has sabotaged the navigational system, and the TARDIS accelerates out of control, heading for the far reaches of the Universe. But there's one side effect which the Doctor wasn't expecting; the turbulence causes one of the Rani's sample jars to fall and shatter, and time spillage from the accelerating TARDIS causes the embryo inside to begin growing. The Master and the Rani are trapped, pressed against the walls of the console room by the acceleration as the T-Rex grows to adulthood...