It is the year 1930, and noted biscuit magnate Hepworth Stackhouse is on his way to the seaside resort of Nutchurch, having been ordered by his doctor to relax and take the sea air. On the train to Nutchurch, he encounters a rather odd man named Percy Closed, who behaves as if he has been carefully instructed in proper British mannerisms, and who is carrying an inexplicable technological device in his carry-on luggage. Stackhouse is intrigued despite himself, and upon sighting Closed in Nutchurch some days later, he decides to follow him and investigate, sure that the man is a foreign spy of some sort. He follows Closed to an isolated cove, where Closed greets a woman emerging from a solitary bathing hut which Stackhouse could have sworn was empty. As Closed and the woman depart, Stackhouse approaches the hut to investigate further -- but a noxious vapour spews out of the hut and envelops him, enslaving him to the will of Zodaal...

Some months later, London is suffering through an unseasonable heat wave, and mystery novelist Felicia Chater is preparing to dine with an old family friend, Colonel Radlett, to determine whether he would make a suitable husband. She is interrupted, however, by a shrill whine from her new next-door neighbour’s home, which continues throughout the day. Her neighbour is Percy Closed, who is attending a meeting of the Circle elsewhere in London, entertaining his friends with his wit, his skill at the piano, and his story of meeting the famous Stackhouse. The transmission from home is forwarded to his personal receiver, but he nervously laughs the signal off as unimportant; however, he can’t laugh off his discovery that Harriet Kipps has smuggled an anti-gravity belt with her and has been using it to float over London at night. Despite her pleas, he confiscates the belt, intending to dispose of it when an opportunity presents itself. Meanwhile, the infamous assassin Julia Orlostro is summoned to the home of Hepworth Stackhouse -- where she finds that both Stackhouse and his manservant Orlick are undead, their bodies animated by an infernal green vapour. Despite her horror, she has accepted a commission from Stackhouse, and honour demands that she carry it out by kidnapping the noted seismologist Heath Porteous and taking him to a warehouse by the docks.

The TARDIS materialises in London when the Doctor deactivates the Randomiser, intending to return his overdue library books before they become overdue. As he prepares to set off, the console picks up the signal being sent to Percy’s home, and when Romana investigates, she detects the presence of time-warmed chronons in the atmosphere, proof that there is an unshielded time corridor somewhere in the vicinity. The Doctor refuses to investigate, and Romana thus sets off on her own, leaving K9 in the TARDIS. On his way to the library, the Doctor realises that he hasn’t finished reading the books yet and makes a detour to a Lyons tea room, where he sees Julia kidnapping Porteous and attempts to intervene. Julia gets away when London is rocked by an unexpected and geologically improbable earth tremor. The Doctor, having recognised Porteous, realises that the kidnapping of a seismologist at such a time can hardly be a coincidence, and returns to the TARDIS to fetch K9 and investigate. Meanwhile, Romana runs into Colonel Radlett while tracing the source of the signal which she’d detected, and the Colonel is instantly infatuated with her; however, he must keep his appointment at Felicia’s, even though he subsequently has trouble concentrating on her conversation. For her part, Felicia finds his tales of hunting and warfare quite tedious -- and when Percy returns home, Felicia sees him and instantly falls head over heels in love. Romana, however, has finally traced the source of the signal, and she confronts Percy outside his home and demands an explanation for his access to anachronistic technology.

Julia takes Porteous to Stackhouse’s warehouse, where the reanimated bodies of dead labourers have constructed a seismic stimulator capable of destroying the world if programmed correctly. Stackhouse’s lawyer, the jittery Alfred Woodrow, orders Porteous to do so, and threatens to destroy his prize rosebushes if he refuses. As the cowed Porteous sets to work, Woodrow explains to Julia that he was hired some months ago to sell off all of Stackhouse’s belongings as quickly as possible in order to fund his work. Greedy for more money, he agreed to arrange the “accident” that killed these workmen, only to watch in horror as they rose from the dead. Stackhouse set them to work, feeding them animal brains for sustenance, and Woodrow continued to serve him for fear of what would happen if he refused. Meanwhile, the odour that is Zodaal senses a powerful alien mind in the vicinity, and departs from Stackhouse’s carcass to find and destroy this potential threat. It homes in on the Doctor just as he and K9 track down Romana to Percy’s home, but as the noxious vapour attempts to smother him, the Doctor closes off his mind and body to Zodaal’s attack. The commotion draws the attention of Percy, Romana and Colonel Radlett, and Zodaal retreats, frustrated.

Romana takes the Doctor into Percy’s home, ignoring the Colonel’s blustering attempts to help her. Inside, she and Percy explain to the Doctor that Percy represents an agency from Earth’s future which is using an abandoned alien time corridor to offer its clients peaceful retirement in the past. Something now appears to be blocking the corridor, preventing Percy from deciphering what appears to be an emergency signal from the agency. When K9 announces that the gaseous being which attacked the Doctor was composed of the rare compound radmium, which is found only in certain the cores of double-spiralled stars, the Doctor theorises that something has followed Percy and his Circle through the corridor. It will have to be shut down while he investigates. Percy reluctantly directs Romana and K9 to the terminal in Nutchurch. The next morning, as they set off, Colonel Radlett returns, having decided to sweep the beautiful Romana away from her unsuitable bohemian friends. She accepts his offer of a ride to Nutchurch, but on the way they encounter Orlick, who has been sent by Stackhouse to destroy the corridor and prevent any further interference in his plans. Orlick senses the threat posed by Romana and attempts to kill her, but K9 fires back at his car, destroying it and burning his body beyond recognition. Romana explains to the bemused Colonel that they are on a secret mission for the government, and when it becomes apparent that Percy lied about the location of the corridor’s terminal, the Colonel gallantly offers his assistance in helping Romana find whatever it she’s looking for. Meanwhile, a passing police sergeant finds the wreckage of Orlick’s car -- and is possessed by the essence of Zodaal which formerly occupied Orlick’s body.

Felicia invites Percy over for a game of bridge, hoping to get to know him better. As the Doctor has finished work on a device with which to locate the source of any further disturbances, he accepts on the reluctant Percy’s behalf. However, unaware that Felicia is infatuated with him, Percy makes an excuse and returns home in order to warn the rest of the Circle about the Doctor’s involvement. Meanwhile, Stackhouse orders Julia to kill the Doctor, but Julia, realising that the Doctor must therefore pose a threat to Stackhouse, decides instead to question him, learn of Stackhouse’s weakness, and destroy him -- and then use the seismic stimulator to hold the world to ransom. When she follows Stackhouse’s directions to Percy’s home, she finds Percy alone, and kidnaps him, erroneously believing him to be the Doctor. She orders Woodrow to report the Doctor’s death to Stackhouse, but Stackhouse can still sense the Doctor’s living mind and thus knows that Woodrow is lying. He therefore kills Woodrow, reanimates him with more of the essence of Zodaal, and sends him to finish the job.

Romana and the Colonel locate the bathing hut which serves as the time corridor’s terminal, but are then attacked by the possessed policeman and are forced to retreat, unprotected, into the corridor. K9 follows them and seals the door of the hut as he goes, and when the policeman tries to open it he is electrocuted by the hut’s defense systems. Romana, K9 and the Colonel recover from their ordeal to find themselves aboard an alien spacecraft, stuck in the middle of the time corridor. The remains of a wolf-like being have been strapped into what appears to be a transfer device of some kind, and Romana realises that the ship’s occupant bound his mental patterns to the radmium used in the ship’s drive, thus becoming a sentient gas and escaping his prison here. She and the others will never survive another unprotected trip through the time corridor, however, and are thus trapped on the ship. The Colonel still doesn’t understand what’s going on, and when he hears a voice which seems to come from behind a row of bottled jars, he begins to move them aside -- only to realise that the voice is in fact coming from within the jar he is now holding. In his shock he drops the jar, shattering it, and releasing a cloud of sentient radmium which attempts to possess him.

Julia searches Percy and finds the anti-gravity belt which he had confiscated from Harriet Kipps earlier. She soon realises that he is not the Doctor, and forces him to call his home and summon the real Doctor here. Just as the Doctor and Felicia arrive, the possessed Woodrow arrives and attempts to kill them all. Believing that the Doctor is outmatched, Julia makes her escape with the anti-gravity belt, but the Doctor manages to trap Woodrow, sets Percy’s communicator to overload and flees with him and Felicia as the communicator explodes, incinerating Woodrow. Percy informs the Doctor that Julia and Woodrow had mentioned working for Stackhouse, and the Doctor thus visits Stackhouse’s mansion to search for further clues. There, he is attacked by a living hand animated by the essence of Zodaal, but defeats it and escapes with the address of the warehouse where the seismic stimulator is located. There, Porteous has finished his work, and Stackhouse throws him into a storage room in the cellar to contemplate what he has done. Meanwhile, Julia returns to the warehouse, believing that the Doctor is dead and realising that it is up to her to prevent Stackhouse from destroying the world.

To save the Colonel’s life, K9 offers himself up as an alternate vessel for the radmium creature, as he is able to maintain his personality in a backup system while playing host. The creature identifies itself as a part of the mind of Zodaal, a scientist from the Phryxus system who faced criminal charges for conducting unethical experiments upon animal subjects. He escaped and tried to flee in an experimental spaceship, but its prototype radmium drive failed and left him stranded in this time corridor. He soon devised an escape plan, but realised that it would require a great many deaths, and therefore, when he bound himself to his star drive’s radmium power source, he separated his senses of humour and decency and sent the rest of himself off to do what was necessary. Romana agrees to transport the rest of him to Earth, where the essences will be reunited and the complete Zodaal will be restored; in exchange, Zodaal agrees to stop his other self from carrying out his plan to destroy the Earth. He and K9 work together to calculate a safe path through the time corridor, and Zodaal then reluctantly allows Romana to return him to his storage bottle until she can reunite him with the rest of himself.

Stackhouse activates the seismic stimulator, and the Earth begins, slowly at first, to shake itself apart. The unnatural heat wave caused by his earlier experiments is broken by a thunderous storm. The Doctor, Percy, and Felicia arrive at the warehouse and confront Stackhouse, only to be taken prisoner. Percy and Felicia are locked up with Porteous, while Stackhouse prepares to transfer the essence of Zodaal into the Doctor’s body, explaining that Zodaal originally intended to use the energy from the Earth’s destruction to bind himself permanently to a host of strong mind and body. As his original choice of host has disappeared, he will use the Doctor instead -- but upon closer examination the Doctor’s alien brain proves unsuitable. Meanwhile, Julia releases Percy and Felicia and questions Porteous, who claims that it is too late to reverse what he has done. Percy flees from the warehouse, but Felicia attempts to release the Doctor and is recaptured. Julia tries to shut down the seismic stimulator, but triggers a booby trap and is recaptured. Zodaal has his chosen host back again...

Romana, K9 and the Colonel emerge from the bathing hut to discover that when the policeman was electrocuted, the essence of Zodaal was forced out of his body. They return to Percy’s house through the raging thunderstorm, only to be captured by the other members of the Circle, who are armed with futuristic weapons and have realised that their friend has gone missing. Romana explains the situation to them, and they help her to repair the Doctor’s seismic detector and track down the source of the disturbances to the warehouse. There, they find the miserable Percy lurking outside, and he tells them what has happened. Romana tells the others to wait for her, and enters the warehouse and releases the better half of Zodaal; but once reunited, he decides to carry out his plan anyway, telling Romana that she’s just helped him to see the funny side of it. Stackhouse locks up the Doctor, Romana and Felicia with Porteous, and pillages K9’s data banks, drawing out everything he knows about time travel. He then operates on Julia, connecting her brain directly to the device which will enable him to possess her once the Earth is destroyed.

K9 manages to send a distress signal to the rest of the Circle, and they storm the warehouse and attack Zodaal’s zombies. The Colonel fights alongside Harriet Kipps as Percy rushes past the front lines and frees the Doctor. The Doctor snaps Porteous out of his despair, and Porteous reveals that he carries a portable seismic monitor with him at all times, from fear that his jealous scientific colleagues will attempt to sabotage his experiments. The Doctor rewires the monitor and takes it to Stackhouse, claiming that the readouts prove that the seismic stimulator is useless. Furious, Stackhouse grabs the monitor to see for himself, and it electrocutes him, forcing the essence of Zodaal out of his body. As the vapour pours into the escape pod, the Doctor, realising that he is too late to save Julia, fulfils her last request and places her gun in her hand. Zodaal’s ship then launches, escaping from the Earth, but K9 bypasses the seismic stimulator’s security grid, enabling the Doctor to recircuit it. He and his friends flee as the warehouse explodes, saving the world. The Colonel and Harriet Kipps, having fought beside each other and survived, have now fallen in love, and the Doctor, Romana and K9 leave their new friends to their new lives as they set off on their travels once again. Meanwhile, Zodaal discovers too late that Julia has shot herself, leaving Zodaal to drift helplessly through space for all time, without a host body to pilot his spacecraft.