On a standard flight from New York to London, Concorde Golf Victor Foxtrot is nearing Heathrow Airport when its signal begins to break up. Before long all trace of the aircraft is lost - the Concorde has disappeared. Arriving at Heathrow shortly afterward, though still grieving for the death of Adric (Earthshock), the Doctor, Nyssa and Tegan, are enlisted by Department C19 to help in the investigation of the missing craft.
The trio board a similar Concorde, Golf Alpha Charlie, and follow the same flight path to try and discover the cause of the disappearing Concorde. The TARDIS is stowed on board. Stapley, the Captain of the Concorde, and his senior crew welcome them aboard. The Doctor finds traces of disturbance and, although they arrive safely at Heathrow (as shown in the picture above), they discover that they have travelled 140 million years into the past. The crew believe they have landed in modern Heathrow until the Doctor and Nyssa urge them to challenge this perception and realise the reality of the empty landscape, which has been distorted by huge amounts of psychokinetic energy. They soon spy Victor-Foxtrot on the empty plain, and an impressive citadel beyond it in the far distance, with the remains of an alien spacecraft nearby. When the Doctor and his friends discover the crew and passengers of the first Concorde they are moving his TARDIS toward the Citadel on the instructions of an alien entity, all totally immersed in the illusion of a modern Heathrow. All, that is, apart from one passenger, Professor Hayter, who has resisted the illusion. Bilton and Scobie, Stapley’s flight crew, are less lucky and succumb to the illusion, heading off for the Citadel with the TARDIS and the other confused passengers. Their progress is marshalled by the Plasmatons - blobs of protein agglomeration from the atmosphere assembled of random particles, which are held together by the same kinetic energy.
The force in charge of this strange domain is Kalid, a seemingly oriental mystic who uses a glowing green globe to control vast amounts of psychokinetic energy and shape the prehistoric landscape of Earth. Nyssa too has a particular empathy with this energy and starts getting visions and voices in her head that are so unwelcome to Kalid that he tries to cut her off from the others with a protoplasmic shield. Tegan stays with Nyssa while the Doctor ventures on to the Citadel with Hayter and Stapley. There they find the crew of Victor-Foxtrot, blindly trying to remove the walls to a sealed chamber. Stapley and Hayter work on trying to free the others from the mental illusion while the Doctor heads to the heart of the Citadel and encounters Kalid. The green-tinged magician has evidently brought a slave force to prehistoric Earth, tapping into the already existent psychokinetic powers of the place. He then channels the energies into menacing Hayter, Stapley and the others to try to secure the Doctor’s cooperation in getting into his TARDIS.
This exertion has broken Kalid’s mental hold over the plasmatons around Nyssa and they disperse. Nyssa and Tegan follow the former’s instincts as they enter the Citadel and they are soon provided access to the chamber that has been closed to Kalid and the mentally deluded passengers. Nyssa acts on instinct and throws an artefact into the centre of a tank-like structure in the centre of the sealed room, and the results are immense. Kalid’s mental channelling is interrupted and he collapses in agony, his disguise falling away to reveal the Master. The renegade is trapped in this time zone, having damaged his TARDIS while escaping from their previous encounter on Castrovalva, and needs a new source of power to escape this time zone. The power in the closed chamber could provide an alternative source, but the Master is frustrated that the passengers are taking so long to break down the walls and access it. He forces the Doctor to give him the key to the TARDIS and steals the craft to try to enter the chamber another way, and the Doctor and Hayter race off to the Chamber to try to reach it first. Their arrival coincides with the Concorde passengers finally breaking through the wall.
Inside the sanctum the Doctor and Hayter are reunited with Nyssa and Tegan. The sarcophagus at its centre contains a being of immense power but a split personality, which has let itself be used by the Master and Nyssa respectively. Nearby are small shrunken bodies which the Doctor identifies as a missing species, the Xeraphin, a race of ancient beings believed destroyed in the crossfire during the Vardon/Kosnax war. Instead, the entire race seems to have transformed itself into a single gestalt intelligence within the tank which has phenomenal psychic abilities. Hayter sacrifices himself to the creature to provide it with a means to communicate and is absorbed into the entity. The Xeraphin manifest itself in the being Anithon, who explains the entity came to Earth in the crashed spaceship on the plains to escape the war, but were so plagued with radiation they shed their bodies and became a single bioplasmic entity. The Xeraphin built the Citadel and planned to re-emerge from the sarcophagus once the radiation danger was over. The Master’s arrival disturbed the balance of the Xeraphin and has caused the gestalt to develop a split personality of good and evil, each side competing for their tremendous power while yearning to become a proper species once again.
As a result of the Doctor leaving the coordinate override switched on, and some sabotage by Captain Stapley, the TARDIS fails to take the Master into the central Chamber. His next gambit is to build an induction loop which he uses to remotely access the sarcophagus and exert his will over it. The bad Xeraphin responds and within moments the sarcophagus is transported into the centre of the Master’s own TARDIS to serve as a new power source. The Master attempts to flee in his ship, taking those passengers still deluded with him as a slave crew, leaving the Doctor and his friends stranded. But due to the earlier sabotage by the Captain, the Master is unable to leave prehistoric earth. After some to-ing and fro-ing over missing parts, the Doctor manages to gain the release of all the passengers and some parts stolen from his own TARDIS in return for the Master getting a new temporal limiter.
There is now a mass departure from prehistoric Earth. First, the second Concorde is made serviceable and transports Stapley, his crew and the passengers from the other Concorde back to Heathrow. The Doctor reverses the track of the time contour and brings the plane back to Heathrow along with his TARDIS. The Doctor programmed the temporal limiter that he provided the Master with to arrive after he did, so when the Master attempts to land, the Doctor's TARDIS is already in the spot. He bounces the Master's TARDIS away from its intended destination, and the evil Time Lord is sent back to modern-day Xeriphas, where the Doctor hopes the Xeraphin will exact their revenge.
In a rush to leave, the Doctor and Nyssa head off in his TARDIS, assuming that now Tegan is back in her beloved Heathrow she will be happy to stay. Her sadness as she sees the TARDIS dematerialise tells a different story.