The Doctor, Nyssa, and Tegan, still mourning for Adric, arrive at Heathrow Airport and learn from Department C-19 that a Concorde plane has mysteriously disappeared out of the sky as it was landing. After dropping Brigadier Lethbridge-Stewart’s name, they place the TARDIS on board another Concorde and join Captain Stapley (Richard Easton) to fly the same landing path. They seem to land normally, but the Doctor determines that they have passed through a Time Corridor to 140 million years in the past. There is an illusion of Heathrow projected by a Psychokinetic Field.
They find the crew and passengers of the missing plane, who believe they are at Heathrow but are enslaved by Plasmatons—humanoid blobs of protein held together by a Psychokinetic Field. Only one passenger, Professor Hayter (Nigel Stock), sees through the illusion. He tells the Doctor they have been forced to work by the mystical Kalid to break into a central chamber at a nearby citadel. As the Doctor leaves to confront Kalid, Shapley and Hayter attempt to wake up the other humans.
Nyssa, with her empathic abilities (If this came up before, I missed it) is able to enter the central chamber and with Tegan (If she has empathic abilities, I never saw them) to look for the power source of the powerful field. Nyssa interrupts the power source, which causes Kalid’s disguise to break down, revealing that he is the Master (Anthony Ainley). He explains that he has been trapped in Earth’s past, his TARDIS damaged, and thinks he can repair it with the power in the citadel. He created the time corridor to get slaves to break into the chamber, but now he can use the Doctor’s TARDIS to enter it. He steals the TARDIS and vanishes.
The Doctor finds that the humans have broken through into the chamber and he joins Nyssa and Tegan inside. They find that the power source is the combined intelligence of many Xeraphin. They too are stranded and, vulnerable to radiation, created the Gestalt Intelligence to survive. But when the Master arrived, he caused that intelligence to develop a split personality. Some of them help the Master, others struggle against him. The Master is unable to materialize the Doctor’s TARDIS inside, but he uses it to create an Induction Loop to transfer the intelligence to his own TARDIS. But he finds that Stapley and Hayter have removed some of the important circuitry.
He tries to take those parts from the Doctor’s TARDIS, but the Doctor proposes a truce. He will provide the parts to repair the Master’s TARDIS if he will drop the Psychokinetic Field. The Master agrees and leaves in his TARDIS. The Doctor puts the freed humans and his companions in one of the Concordes and uses the TARDIS to bring it to Heathrow. However, he programmed the Temporal Limiter he had given the Master to arrive later than himself and is able to prevent him from rematerializing anywhere but the Xeraphin’s home planet. The Doctor leaves with Nyssa, but Tegan is late and is stranded at Heathrow.
Peter Davison was disappointed in the story. He thought it was good, but they had run out of money. The prehistoric landscape was in the studio with a model Concorde in the back, and the Plasmatrons were bits of foam. The viewer numbers for the first episode were great (following the highly popular Earthshock), but by the time they reached the fourth episode, they had lost two million viewers. Approval has dropped even more since, largely because of savage reviews. The Concorde they used was G-BOAC, the flagship of the fleet at the time. Heathrow’s product placement was obvious. To obtain permission to film at Heathrow and in a Concorde, they were given script approval and made some changes. One thing they were careful about was the wholesome image presented by the three actors playing the flight-crew of the Concorde, but they were thrilled with Richard Easton, Keith Drinkel, and Michael Cashman. They had no idea all three were gay. At the end, it looked like Tegan was finally back home, but she turned up again in the next story, under a new contract. The next serial was re-written to include her. Adric’s image appeared briefly to Nyssa and Tegan because Matthew Waterhouse was still under contract.