Speedbird Concorde 192, Golf Victor Foxtrot, en route from New York to London, is making its final approach to Heathrow Airport when something goes wrong. Captain Urquhart is in contact with air traffic controller Horton when the radio signal begins to break up. The transponder signal also becomes intermittent and then goes out altogether. The Concorde has just vanished into thin air...

The Doctor has returned the freighter crew to their own time and seen to the dispersion of the Cyber fleet, but there's one thing he can't fix; Adric is dead, and despite Tegan's pleas even the Doctor is unable to break the Laws of Time to save him. In order to cheer up himself and his companions the Doctor decides to visit the Great Exhibition of 1851, but the TARDIS encounters heavy turbulence, as if it's been caught in the wake of another time machine. The Doctor is forced to materialize to avoid breakup, and both he and Tegan are surprised to find themselves hovering over Heathrow Airport in the mid-1980s. They appear briefly on Horton's radar in the same flight path as Victor-Foxtrot, but the Doctor operates the co-ordinate override to take the TARDIS out of the path of the incoming flights and materializes in Terminal One.

The Doctor pops out of the TARDIS for a moment to check current cricket results in a newspaper, but he and his companions are then accosted by the head of security, Jim Andrews, who demands to know why there's a police box in his airport. The Doctor tries to cover by citing his UNIT clearances and inviting Andrews to check them out with Sir John Sudbury of Department C19, but once Sudbury learns that the Doctor is present he insists that the Doctor be briefed on the disappearance of the Concorde. Despite Tegan's reluctance to get involved and Airport Controller Sheard's scepticism of the Doctor's qualifications, the Doctor attends a meeting and learns what happened. He realizes that it would also explain what happened to the TARDIS if Victor-Foxtrot flew into a time warp, but the only way to be sure is to take another Concorde on the same flight path.

The Doctor, Nyssa and Tegan board the Concorde coded Golf Alpha Charlie, and are introduced to Captain Stapley, first officer Andrew Bilton, and flight engineer Roger Scobie. The flight crew consider the Doctor's theory about a time warp a bit of a laugh, and the Doctor decides to spend the flight in the TARDIS, which has been loaded into the cargo bay. After turning on the artificial gravity he monitors the flight from the console -- and as Alpha-Charlie approaches Heathrow, on the same flight path as Victor-Foxtrot, the TARDIS console registers that it is moving through Time. Horton and Sheard watch helplessly as Alpha-Charlie's transponder signal vanishes from the radar screen...

But much to the Doctor's surprise, Stapley manages to contact Heathrow Airport and land safely. All seems normal when they disembark, until Nyssa catches a brief glimpse of decaying corpses in a stark wilderness. The Doctor realizes that they're being fed a perceptual illusion and urges the others to concentrate on the details of their surroundings. Once they do so the illusion is stripped away to reveal that, although they have indeed landed at Heathrow, they've done it over 140 million years in the past. Stapley, Bilton and Scobie are shaken both by the realization that the Doctor was right and by the condition of Concorde, which they landed on the rough, rocky terrain without even realizing it. The Doctor is more concerned with the source of the illusion -- where there's magic there must be a magician. Indeed, elsewhere the green- skinned magician Kalid is watching events on a crystal ball, and he sends the passengers of Victor-Foxtrot to bring the TARDIS to him.

Something tries to communicate with the Doctor through the plasmic mass, but Kalid angrily breaks the connection and the protoplasm disperses into thin air. The Doctor, recovering, identifies the "creatures" they saw as Plasmatons, nothing more than raw protein manifested from air molecules and psychokinetically animated by the same power that induced their hallucinations. Kalid, meanwhile, places Bilton and Scobie under his spell and sets them to work with the passengers of Victor-Foxtrot.

The Doctor and his companions are contacted by Professor Hayter, a passenger from Victor-Foxtrot. He studies perceptual conditioning at Darlington and was thus able to resist the illusion when he was first brought here; however, he believes that they have been hijacked by the Soviets and brought behind the Iron Curtain, and that the source of the hallucinations is an ultrasonic transmitter of some kind. When Tegan and Nyssa try to tell him that they've gone back in time several million years he is frankly disbelieving. He reluctantly shows them to the citadel from which he'd escaped, but as they approach Nyssa falls to the ground and begins speaking with the voice of the intelligence which is guiding them. The intelligence seems divided amongst itself, and as Nyssa struggles to communicate, Kalid traps her in another protoplasmic shield to prevent her from talking.

The Doctor concludes that the only way to free Nyssa is to reach the source of the power, and Tegan stays with Nyssa while Stapley and the reluctant Hayter accompany the Doctor to the citadel. The Doctor assures them that they have nothing to fear from the Plasmatons, as the energy which created them is now fully occupied in holding Nyssa prisoner. Hayter, who assumes that the Plasmatons are guards in disguise, isn't convinced. Inside the citadel, they find the passengers of Victor-Foxtrot trying to break into a sealed chamber in a mausoleum. The Doctor wanders off, searching for the TARDIS, while Hayter and Stapley try to break Bilton and Scobie's conditioning. Stapley nearly succumbs to the mass hallucination himself, but Hayter snaps him out of it and they are able to free Bilton and Scobie as well.

The Doctor, meanwhile, finds his way to Kalid's chambers. Kalid claims to have learned the magical arts in the deserts of Arabia, but the Doctor isn't convinced. Why has Kalid generated an artificial time contour? Why kidnap the passengers of Victor-Foxtrot as slaves and where did the crashed spacecraft come from? The Doctor soon realizes that Kalid is not the source of the power -- he's just tapping it for his own use. Kalid demands access to the TARDIS, but the Doctor refuses, and Kalid uses the Plasmatons to bring Hayter, Stapley, Bilton and Scobie to his chambers for use as bargaining tools.

The shield around Nyssa evaporates as soon as Kalid withdraws the power. Convinced by some intuition that the Doctor is in danger, Nyssa rushes off to the citadel, followed by Tegan. Kalid sees their approach and tries to fend them off with illusions -- Adric begging for his life, Melkur, and a Terileptil -- but Nyssa and Tegan resist the illusions and proceed onwards. Sealed passages open for them as they go, as if some other force is guiding them. They soon reach the sanctum at the heart of the citadel, the room which the passengers of Victor-Foxtrot have been trying to break into -- and which not even Kalid has been able to penetrate.

Scobie notices that there are no connections between the electronics and the crystal ball, and the Doctor realizes that the electronics are components from the Master's TARDIS. He's stuck here, needing a new source of power for his ship; the time contour was a desperate lifeline which netted him slaves from Victor-Foxtrot, but as they've been unable to break into the sanctum he needs the Doctor's TARDIS. Thanks to Nyssa and Tegan the power has been temporarily depleted, but it will soon replenish itself; but the Doctor believes that the power is working both for and against the Master, and that Tegan and Nyssa may have survived. He hands over the TARDIS key when the Master threatens the others' lives, and Hayter watches in shock as the Master enters the Doctor's TARDIS and dematerializes.

The Doctor reveals that he left the co-ordinate override switched on; this will delay the Master and give the Doctor a chance to get inside the sanctum first. He takes Hayter, who's slowly starting to realize that the Doctor was right all along, but orders Stapley, Bilton and Scobie to stay where they are; the power might return at any moment and the hallucinatory matrix would be too strong for them to resist. The Doctor and Hayter find a very confused group of Concorde passengers in the mausoleum, and with Angela Clifford's help they convince them to finish their work and break into the sanctum. As they do so, the Doctor notices the Master's TARDIS nearby, connected to an induction loop which is already drawing power from within the sanctum.

The passengers break through the wall, and the Doctor and Hayter enter the sanctum to find Tegan and Nyssa recovering. Inside the sarcophagus they find a living organism, and the Doctor wonders why its actions have been so contradictory; first it summoned Tegan and Nyssa and caused them to attack it, then diverted its power to deflect the attack, depriving the Master of power -- while allowing Tegan and Nyssa to survive. As he muses, the creature's power starts to return, and the passengers once again fall under its spell -- and wall up the Doctor and his companions within the sanctum.

The Doctor's TARDIS returns to the Master's chambers, and the irritated Master emerges to collect parts from his crystal ball. Stapley and Bilton slip into the TARDIS and switch about components within the central console, hoping to delay him, but the Master returns and catches them. After taking the components he needs, he sets the TARDIS to dematerialize automatically and departs, leaving Stapley and Bilton stranded as the TARDIS hovers over the citadel like a helicopter. Scobie, meanwhile, follows the Master out of his chambers and meets Angela, who's still trying to resist the illusions. She leads him back to the mausoleum but falls under the Master's spell once again and enters the TARDIS to wait with the passengers. Scobie hides and watches as the Master connects a series of boosters to the induction loop.

Hayter finds a number of shrunken humanoid figures lying nearby, and the Doctor, Tegan and Nyssa recognise the handiwork of the Master. These are bodies of Xeraphin, a legendary race believed wiped out by crossfire in the Vardan/Kosnax war. It appears that the entire race of the Xeraphin has been melded into a single gestalt, the creature in the sarcophagus; its psychic will must be enormous. The creature attempts to communicate through Nyssa, but the Doctor tries to warn her back, as communion with the Xeraphin at full power will destroy her. Hayter takes her place, willing to sacrifice himself for the opportunity of total scientific knowledge, but communion with the Xeraphin destroys his mind, and his body disintegrates into nothing.

Stapley and Bilton's attempt to pilot the TARDIS themselves comes to nothing, and they barely manage to stabilise it again. But as they ponder their next move, Professor Hayter suddenly appears on board and pilots the TARDIS into the sanctum. The Doctor, Nyssa and Tegan are trying to break out when the TARDIS arrives, and when Stapley tells them what happened, they realize that Hayter isn't dead after all but has been absorbed into the Xeraphin telepathic gestalt. The Doctor pilots the TARDIS out of the sanctum and is reunited with Scobie, who tells him that the Master took the passengers from Victor-Foxtrot aboard his TARDIS and departed. The Doctor realizes that as the Master no longer needs the passengers as slaves, he intends to "melt them down" into molecular protoplasm which the Xeraphin nucleus can manipulated. Until the sarcophagus has been fully integrated into his TARDIS, however, the Master will be forced remain in the vicinity, giving the Doctor a brief window of opportunity in which to stop him...

The Doctor sends Nyssa and the others back to Alpha-Charlie to prepare for takeoff, while he and Tegan search the citadel for the Master; but the Master has completely abandoned it and taken everything with him, including the components he'd stolen from the Doctor's TARDIS. They return to Alpha-Charlie, which is in surprisingly good condition after its rough landing. Stapley intends to take the parts he needs from Victor-Foxtrot, but before he can do so the Master materializes his

TARDIS around Victor-Foxtrot and then departs with it, leaving the others stranded with a damaged Concorde and a TARDIS with vital parts missing. The Doctor is unable to return the flight crew to their own time, and the Master is free to roam time and space with the power of the Xeraphin at his command...

Or so it seems until Victor-Foxtrot unexpectedly materializes again. The Doctor investigates and finds the Master waiting impatiently for his arrival; it seems that thanks to Stapley's earlier sabotage the Master has taken the wrong component from the Doctor's TARDIS, and without a functioning temporal limiter he will be unable to travel in Time. The Doctor agrees to a trade -- a working temporal limiter in exchange for the passengers from Victor-Foxtrot and the parts the Doctor needs to repair his own TARDIS. The Master reluctantly agrees, but refuses to release the passengers until he has the temporal limiter.

The Doctor tries to keep the Master waiting to give Stapley and his crew a chance to repair Alpha-Charlie, but the Master grows impatient and threatens to start killing passengers unless the Doctor hands over the limiter. The Doctor does so, and the Master releases the passengers and departs. The Master will now trace back the path of the time contour, arriving in Heathrow Airport with a fully functional TARDIS and the power of the Xeraphin at his command... However, the Doctor admits that he programmed an inhibition factor into the temporal limiter, ensuring that the Doctor will arrive first.

Tegan finally gets to act as a flight attendant, guiding the confused passengers aboard Alpha-Charlie to complete their journey. Stapley and his crew successfully take off despite the rough terrain, and the Doctor programmes their flight computer to put them back on approach to Heathrow Airport. He, Nyssa and Tegan return to the TARDIS and dematerialize -- and thus kick Alpha-Charlie back along the time contour to the twentieth century. The aircraft suddenly appears on Horton's radar, 24 hours after it first disappeared, and Stapley and his crew land safely at Heathrow.

The TARDIS materializes in a parking lot, and while the Doctor waits for the Master to arrive, Tegan and Nyssa watch the planes landing and taking off. Nyssa realizes that Tegan still misses air travel and wonders if she wants to stay. The Master's TARDIS materializes in mid-air, but is stuck in transit as the Doctor's TARDIS is already occupying its landing co-ordinates. The Doctor is thus able to use his TARDIS to knock the Master's back into the space-time continuum and force it to materialize on Xeriphas. The surge of energy will have burnt out the Doctor's temporal limiter, leaving the Master stranded -- and 140 million years after the war, the planet will be habitable again, and the Xeraphin can regenerate.