The TARDIS materializes on a vast alien spacecraft. The time-travellers are observed by a hovering surveillance ball. They become separated. The Doctor and Tegan arrive at the Bridge, where a green-skinned commander introduces himself as Monarch, ruler of Urbanken (Stratford Johns). He is accompanied by The Minister of Enlightenment (Annie Lambert) and the Minister of Persuasion (Paul Shelley). The leader is interested in news of Earth, as they are headed there. Enlightenment and Persuasion assume human shape, dressed in clothing based on Tegan’s drawings.
The time-travellers are reunited and treated as guests. It appears that there are four human cultures on the ship. The philosopher Bigon (Philip Locke) is an ancient Greek, Lin Futu (Burt Kwouk) is the leader of Chinese Mandarins, Princess Villagra (Nadia Hammam) is Mayan, and Kurkutsi (Illario Bisi Pedro) is the leader of a tribe of Australian Aborigines. The Urbankans, it seems, have been to Earth many times. This time they carry three billion of their species in microscopic form, so they can settle permanently on Earth to escape a black hole at home. They will arrive in four days.
The Doctor learns that the Urbankans plan to release a toxin which will wipe out the human species, and he learns that the humans on board are not descendants of abductees, but the original humans who have been converted to androids, as are the Urbankans themselves. They remember the flesh-time when they were flesh and blood. The four leaders of the passengers have been given circuits for reason, but this can be taken away. Bigon rebels against Monarch and his neural circuit is placed in a box for 100 years as punishment. Monarch strip-mined and destroyed Urbanka for metals to build his ship and plans to do the same to Earth. Moreover, if he can exceed the speed of light, he believes he can go back to the beginning of time and become God.
Adric rather likes and admires Monarch and the Doctor is appalled. It takes the hard truth to bring Adric around. With the help of androids, the Doctor foments a rebellion. Enlightenment and Persuasion are de-circuited, and Monarch, who is still organic, is exposed to his toxin and shrunk to tiny size. The Doctor puts a helmet over him. The humanoids decide to move on to an uninhabited planet, while the TARDIS dematerializes. Nyssa falls to the floor in a faint. This was the first 5th Doctor story filmed, though not the first broadcast, and it wasn’t bad. It was genuine science-fiction. There was even a spacewalk, of sorts, though that was probably the least convincing scene. Peter Davison pretty much walked right in and took over the lead. When the TARDIS appears on the spaceship, he comes out and wanders about, fascinated. The aliens are watching him through the surveillance device, and he smiles. A female alien says, “What a lovely smile!” Well, it was the British audience watching and his smile said, “Hello. I’m the Doctor.” Nyssa was supposed to be written out of the series, but Peter Davison insisted she was the best companion for the Doctor. Nyssa collapsed at the end so she could be ill in the TARDIS for a while, as the next story had already been written without her. It seems to me that Nyssa is a lot more useful than Tegan and more loyal than Adric, who tends to cozy up to the Doctor’s enemies. Frankly, I don’t see why the Doctor can’t just drop Tegan off at Heathrow Airport as he promised and be done with her.
I suppose the answer is that this Doctor is a generous soul and all three of these companions are orphans and lost in time and kind of broken. Adric was a rebel in an incompetent and rigid society, who lost the brother he admired and his whole society buggered off for another planet, leaving him behind with nothing to his name but a gold star for mathematical excellence. He’s not even from the same universe as the rest of us. Tegan was just trying to get to work when she got a flat tire and wandered innocently into a police box, got lost inside, and was taken all over space and time essentially against her will. And the Master turned her beloved aunt into a doll. Nyssa was an aristocrat with a fine life ahead of her, when the Master took over her father’s body, jettisoning his mind, and then erased her entire planet, making her the Princess Leia of Doctor Who.