Kalid has the TARDIS dragged in. He explains that he is a wizard from Arabia who was contacted by the "great power" which has allowed him to enter this realm and do the bidding of the power. The Doctor asks bluntly what Kalid wants. The Master arrives and explains that the TARDIS is needed by the power - and the power is willing to pay for its services.
Nyssa can see Tremas and Tegan can see Aunt Vanessa. Whoever it is, they explain that the Xeraphin escaped the destruction of their planet by becoming a mass of pure thought. However, every so often there is a weak point in space and time that makes them vulnerable to reality. It is such a moment, and when Concord arrived, the realm they are in reacted to all these new non-Xeraphin minds.
Because of all the dark aspects of human nature, a rift has formed between light and dark Xeraphin and the forces have been trying to guide the TARDIS crew since they arrived - though to different ends. Aunt Vanessa urges Tegan to snap the crew out of their trance as the plane is almost ready for take-off. When the passengers are aboard, they must leave at once before the window closes.
Kalid notices Tremas talking to Nyssa and uses his powers to freeze Nyssa in place to stop the good Xeraphin from revealing his plans. Meanwhile, Adric explains to the Doctor that the passengers were brainwashed so they could be used to break into the dome. Inside is the core of the Xeraphin intelligence. Once released they can return to the physical universe and become all powerful.
Kalid explains that he is willing to let the passengers and Concord go in return for using the TARDIS to travel inside the dome and collect the intelligence. The Doctor agrees and Adric calmly points out that he has made mistakes before. But which Xeraphin is talking?
The passengers and crew, lead by Tegan return to the Concord. The Doctor tries to play for time but Kalid insists that time is running out. Soon the window will close and when it does not even the TARDIS can escape. They should do their business at once.
The Doctor lets Kalid inside the TARDIS as Tegan runs back for Nyssa, only to find she has disappeared. The Doctor tells her to wait for him at the "airport". He and Kalid travel to the inside of the dome with Professor Haytor, but find Nyssa is already there, possessed. She picks up a boulder and hurls it at the intelligence.
There is a mighty flash and the intelligence vanishes. Kalid collapses and dissolves - he was a plasmaton, a physic projection of the evil Xeraphin determined to win and given a false personality. But now the good Xeraphin and bad Xeraphin are balanced out, using all their energy to attack and defend itself. Nyssa collapses and the Doctor and Haytor drag her inside the TARDIS.
With the balance restorted, the zigurat and the false Heathrow around it begin to dissolve. Stapley and the pilots power up the plane and the TARDIS materializes to pick up Tegan and drop off Haytor, who thanks the Doctor for the adventure and flees for the airplane as fast as he can. The Doctor decides they must leave at once.
Tegan drops a bombshell. She has checked with Stapley - this is the right date and Concord is heading for Heathrow. She's one step from home and can't bring herself to miss it. The Doctor insists it's safer to go by TARDIS but, with a surprising lack of malice, Tegan reminds the Doctor of Adric. She isn't blaming him, just making a point.
The Doctor watches, shattered, as she turned and runs off through the airport as it dissolves into mist. He then returns to the TARDIS and sets the time machine in motion. It vanishes just as the airport finally vanishes and Corcord flies off into reality.
Aboard the ship the Doctor tends to Nyssa as she recovers. He glosses over Tegan's whereabouts, but when pressed admits she's left. He also reveals he doesn't know whether or not she escaped the realm or died before she reached Concord. Nyssa is devastated, and the Doctor suddenly collapses.
Suddenly, he's dangling from a wire above the Pharos Project. He hears Nyssa calling for him, and then he... lets go.
The Doctor collapses onto the floor of the TARDIS. Nyssa tries to revive him, but he seems to be in a coma. As Nyssa looks around, realizing how alone, she is, the police box spins off into the void.
END OF THE STORY
NEXT: THE TIME OF OMEGA.