On the planet Karfel the small population is ruled in a rigid hierarchy, at the apex of which is the Borad, a sadistic and despotic ruler. The Borad has never shown himself in person, only via security monitors which reveal him to be a dignified old man, but something in his manner does not ring true. Fear is enforced rigidly through the policing of androids; and all rebels – such as Aram, Gazak and Tyheer – are dealt with either by summary execution or despatch and death via the Timelash - a permanent, and ultimately fatal, exile down a corridor of Time and Space. Below the Borad is the Maylin, in effect a proxy mayor figure, who is the most senior of the five Counsellors of Karfel. These Counsellors act more as ciphers than people of true counsel, and one of them, Mykros, has grown unhappy with the rule of the Borad. Since the Borad came to power their people have become disillusioned, rebellious and miserable, and their former allies, the Bandrils, are posed to invade. The Bandrils threaten war after the Borad rescinds the grain supply treaty which underpinned the relationship between the two civilisations.
Mykros determines to discover the truth and follows the Maylin, Renis, into the Borad’s power chamber. The unhappy Maylin is transferring the power supplies of the Karfelons into the Borad’s personal system, despite the danger to his own wife, who is recovering from hospital surgery. Renis finds Mykros and gives him his blessing in rebellion. However, the Borad finds out and metes out the usual punishment: the Maylin is aged to death in a deadly beam while Mykros is sentenced to the Timelash. Before he can be sent in, however, Vena, Renis’ daughter and Mykros’ lover intervenes to plead for his life. When this fails, she steals an amulet conferring the power to pervert the energy supply from the new Maylin, the sycophantic Tekker, and accidentally falls into the web of the Timelash herself.
The arrival of the TARDIS presents Tekker with an opportunity to try and retrieve the amulet. The clever Maylin greets the Doctor and Peri as favoured guests, but the Doctor is suspicious of a Karfelon society that has made huge scientific leaps in a short space of time and that does not permit mirrors. When the Doctor refuses to venture into the Timelash again, Tekker explains that Peri has been taken hostage to ensure his co-operation in retrieving the amulet. She has been taken to the caves of the Morlox (possibly a reference to HG Wells 'The Time Traveller', where the morlocks are degenerated cannibalistic humans), large lizards indigenous to Karfel, where her captors hope she will die. Yet luck is as ever on her side and some Karfelon rebels. Katz and Sezon, kill one of the creatures threatening her and take her into their company. However, they are soon attacked and captured by a patrol of guards.
To protect Peri the Doctor returns the TARDIS into the Timelash and calculates the normal path of the Timelash would be to 1179 on Earth, but the interference of the TARDIS caused her instead to end up in Scotland in 1885. When the Doctor arrived he finds Vena, the amulet and a justifiably agitated young man named Herbert. All three depart on their return journey to return the amulet – which is all Tekker cares about when the TARDIS arrives back in the Council Chamber. The Doctor, Vena and Herbert are rounded up with the rebels Mykros, Sezon and Katz and condemned to the Timelash.
Tekker has meanwhile fled to the Borad, and blames the setback on the last remaining loyal Counsellor, Kendron, whom the Borad executes. Tekker remains at the side of the Borad, now revealed to be a hideous amalgam of human and Morlox. Together they watch on a screen as Peri is brought into a cave and strapped down while Morlox gather to feed. A canister of the chemical Mustakozene-80 is placed nearby, which has the ability to fuse together different tissue as one creature. It seems the Borad has taken a liking to Peri and wishes to mutate her like himself. The Doctor arrives to confront Tekker and the Borad, recognising the latter as Megelen, a crazed scientist he encountered on his previous visit to Karfel and exposed to the Counsel for unethical experiments on Morloxes. It seems one of those experiments has now gone wrong, and Megelen wishes to replicate its effect to create a partner. His plan has been to provoke a war with the Bandrils that will result in their use of bendalypse warheads which will wipe out all the Karfelons – but leave the Morlox and himself alive – allowing him to repopulate the world in his own image. This revelation prompts Tekker too to rebel, but he is swiftly aged to death. The Doctor then uses a Knotron Crystal to deflect Megelen’ beam back at him, killing the mutant in his wheelchair.
Herbert now helps the Doctor rescue Peri from the Morlox. They return to the Council Chamber where Mykros and Vena have identified a Bandril invasion fleet armed with bendalypse warheads which is close to Karfel. The Bandrils are suspicious of the Doctor’s attempts to intervene and prevent a missile strike, causing him to take drastic action. The Doctor materialises the TARDIS in the path of the incoming warhead, risking his own life to save Karfel. He does so successfully and returns to Karfel to find Megelen returned from the dead and threatening the Council Chamber – or rather the other one was a clone of this original. Megelen is made unbalanced by the image of himself in a boarded up mirror, revealing the reason he hid himself away, and in this state is thrown into the Timelash by the Doctor, where he may have ended up as the Loch Ness monster (The doctor says "he may be seen from time to time").
The epilogue sees the Doctor and Peri depart Karfel to return Herbert – or H. G. Wells as he is known – to his own time.