Season nineteen of Doctor Who introduces Peter Davison in the title role, and gives him two opportunities to square off against the new Master. The Cybermen also make a return appearance, and tragedy strikes as one of the Doctor’s companions is killed.
The fall from the radio telescope induces a regeneration in the Doctor. While Nyssa and Tegan Jovanka help him into the TARDIS, Adric is kidnapped by the Master and replaced with a duplicate created through the block transfer computation process. The duplicate Adric sets the coordinates for the Big Bang while the Doctor searches for the Zero Room. He unravels his long scarf so as not to lose his way in the labyrinthine corridors; then resorts to leaving his shoes and waistcoat as well.
The Doctor: Now, ordinary spaces of course show up on the architectural configuration indicators, but any good Zero Room is balanced to zero energy with respect to the world outside its four walls… or however many walls it has. There was a very good polygonal Zero Room under the Junior Senate Block on Gallifrey, widely renowned for its healing properties. Romana’s always telling me I need a holiday.
Adric: But Romana’s gone, Doctor.
The Doctor: Gone? Really? Did she leave a note?
Adric: We said goodbye to her at the gateway. Don’t you remember?
The Doctor: Well, if we did, we did. This should get you back to the console room when the time comes.
Adric: Are you sure you’re all right?
The Doctor: There are strong dimensioning forces this deep in the TARDIS -- tend to make one a bit giddy.
Adric: And the regeneration?
The Doctor: I don’t know. I can feel it isn’t going to be as smooth as on other occasions. The sooner we get to this Zero Room place, the better, eh?
While wandering the corridors, the Doctor comes upon a room filled with cricketing paraphernalia, such as hats, clothing, and plaques; suggesting that at some point in his travels, he played quite a bit of cricket and collected a room full of stuff. It is from this room that he gets most of his new outfit, finding his frock coat hanging in the hallway outside. Meanwhile, Nyssa and Tegan search for him.
Nyssa: This part of the TARDIS can’t have been used for centuries.
Tegan: It does go on and on!
Nyssa: Deeper and deeper.
Tegan: Yes, I get that feeling, too -- that we’re going downwards!
As the TARDIS hurtles back into the Big Bang itself, the cloister bell rings continuously, warning of the imminent danger as the environment begins to exceed the engineering tolerances. The adrenaline and neuropeptides surging through the Doctor’s brain allow him to come up with a solution despite his regeneration crisis. They escape the gravity field by using the architectural configuration system to jettison one quarter of the TARDIS interior. However, the Master had a contingency plan that takes them to the city of Castrovalva on a planet in the Andromeda galaxy, a place where space folds back on itself in maddening ways. The entire city is an illusion created by the block transfer calculation process as a trap, perhaps drawn from Adric’s memories or imagination. When the power web is disrupted, local space begins to fold in on itself, preventing the Master’s TARDIS from leaving. He is apparently trapped in the city when it disappears forever.
From “Four to Doomsday”
The Doctor attempts to materialize at Heathrow Airport on February 28, 1981 so Tegan can catch her flight and not lose her job as a stewardess, but due to a massive magnetic field shift, it arrives instead on a spacecraft. The Doctor soon meets three aliens in the control center. The Doctor lets slip that the TARDIS runs on archon energy.
Monarch: And you are?
The Doctor: I am the Doctor.
Enlightenment: A doctor? Of what?
The Doctor: Of everything!
Tegan displays quite a talent for drawing fashion plates, suggesting that she may have taken the job with the airline to further her dream of being a fashion designer. She also demonstrates an ability to speak the language of the Australian aborigines. Nyssa explains that she is an expert in bioengineering and cybernetics.
When the TARDIS materializes on the planet Deva Loka, Tegan’s mind is invaded by a psionic creature called the Mara. The Doctor says he is familiar with legends of the Mara, and these stories help him defeat the creature. Tegan wants to make sure she is completely free of the Mara’s influence, but the Doctor doesn’t answer.
From “The Visitation”
Once again attempting to deliver Tegan to Heathrow Airport, the TARDIS arrives at the right place, but 300 years early, due to a fault in the lateral balance cones. The Doctor discovers evidence of an alien spacecraft having crash-landed in the area. He learns the aliens are fugitive Terileptil prisoners. While holding the Doctor prisoner, the Terileptil leader destroys the sonic screwdriver with his blaster.
From “Black Orchid”
Tegan decides to stay with her new friends aboard the TARDIS for a while, rather than trying to get back to her job. However, the TARDIS materializes once again on Earth, on June 11, 1925. The Doctor is invited to play in a cricket match at the Cranleigh estate when he is mistaken for the friend of a friend. By a weird coincidence, Nyssa meets her physical double in Charles Cranleigh’s fiancée, Ann Talbot. The time-travelers enjoy a break from their adventures when asked to stay for the annual fancy dress ball. However, they soon become entangled in a murder mystery concerning the Cranleigh family’s dark secret, the demented brother George. They spend a day or so to attend George’s funeral. Upon leaving, they are given their party costumes as a farewell gift.
Adric is sulking in his room, which is full of souvenirs from his travels. He tells the Doctor he wants to return to his people on their adopted world of Teradon in E-Space. The Doctor refuses to take the TARDIS back into the other universe because of the great danger involved. They argue and the Doctor sets the ship down on Earth in 2526 so Adric can try to calculate a course through a CVE and the Doctor can go for a walk. He is clearly upset over Adric’s decision to leave. They find they have wandered into a battle between military personnel and killer androids. After destroying the androids and deactivating a powerful bomb, the Doctor apologizes and Adric says he has changed his mind anyway, as there is nothing for him among his own people. Tracing the signals received by the bomb to a space freighter en route to Earth, the Doctor uncovers a Cybermen plot to destroy the planet by using the freighter as a giant missile. The Cybermen commandeer the TARDIS, forcing Adric to remain aboard the freighter. His tampering with the alien control system sends the freighter hurtling backwards through time. The crew abandons ship, but Adric decides to remain behind to crack the final logic code. The Doctor uses Adric’s badge for mathematical excellence to jam the Cyber Leader’s chestplate and then destroys it with its own blaster. However, the freighter crashes to Earth, wiping out the dinosaurs and killing Adric.
The Doctor: The crew of the freighter safely returned to their own time.
Nyssa: Cyber Fleet dispersed.
Tegan: Oh, great! You make it sound like a shopping list, ticking off things as you go. Aren’t you forgetting something rather important? Adric is dead!
Nyssa: Tegan, please.
The Doctor: We feel his loss as well.
Tegan: Well, you could do more than grieve! You could go back!
Nyssa: Could you?
The Doctor: No.
Nyssa: Surely the TARDIS is quite capable of --
Tegan: We could change what happened if we materialise before Adric was killed!
The Doctor: And change your own history?
Tegan: Look, the freighter could still crash into Earth -- that doesn’t have to be changed. Only Adric doesn’t have to be on board!
The Doctor: Now, listen to me, both of you! There are some rules that cannot be broken even with the TARDIS! Don’t ever ask me to do anything like that again! We must accept that Adric is dead. His life wasn’t wasted; he died trying to save others, just like his brother Varsh. You know, Adric had a choice. This is the way he wanted it.
The TARDIS finally materializes at Heathrow Airport where the Doctor learns a Concorde aircraft has vanished without a trace. The Doctor circumvents airport security by invoking his UNIT clearance. He suspects the plane encountered a time warp, and recreating the phenomenon, they find themselves 140 million years in the past, where they find a wizard who turns out to be the Master in disguise, having somehow escaped from Castrovalva and gotten stranded on pre-historic Earth. He cannibalized his TARDIS to create the time corridor. The Master has apparently exhausted his dynamorphic generator and plans to absorb the amalgamated consciousness of the Xeraphin race as a new power source. The Xeraphin had to meld to survive an aborted colonization attempt. The Doctor manages to strand the Master on Xeriphas, where the Xeraphin will be able to regenerate their physical forms. The TARDIS materializes back at Heathrow where Tegan wanders off, apparently to say goodbye, but the Doctor beats a hasty retreat from the authorities, leaving Tegan behind.