At the behest of the Black Guardian, Turlough (Mark Strickson) sabotages the Space-Time Element of the TARDIS, causing parts of it to dissolve. As the instability threatens to engulf Nyssa’s room, a door opens, and the Doctor tells her to go through. The TARDIS, saving itself, materializes aboard a spaceship heading no-one knows where. The Doctor and Nyssa explore and meet two raiders: Kari (Liza Goddard) and Olvir (Dominic Guard) who are there to steal the ship’s cargo.
When the raiders’ ship suddenly departs, abandoning Kari and Olvir, it becomes clear that the spaceship is a Lazar-ship, carrying victims of Lazarus, a leprosy-like disease, to the space station called Terminus. Terminus Inc. claims there is a cure there, but no-one has been known to return. Nyssa becomes separated from the Doctor and contracts the disease. She is taken away. Terminus is run by guards in ornate armour called Vanir. They are slaves, kept alive by a drug called Hydromel, supplied by the corporation. For Doctor Who, this is a seriously dark story.
Terminus, the Doctor determines, is at the centre of the universe, which news he finds unsettling in the least. Nyssa is turned over to a Garm, a giant bipedal dog who takes her to a chamber and exposes her to radiation. The Doctor and Kari find the control-room and discover it is a time-ship. Its fuel became unstable long ago and was jettisoned in the Time Vortex, where it exploded creating the Big Bang, called Event One by the Time-Lords. The ship was hurled billions of years into the future it created. There is still one tank of unstable fuel left, and the automatic countdown is on to jettison that too, which will no doubt end the universe it began. You’ve got to hand it to Doctor Who for really big threats.
Nyssa awakes to find herself cured, but many others are not so lucky. The Garm knows this but is unable to change the treatment because of its programming. With the Garm’s aid, the Doctor delays the countdown, disables the computer and cuts the engine wires, then sets the Garm free of his restraints. Nyssa strikes a bargain: synthesize hydromel to break the company’s hold over the staff, change Terminus from a Leper Colony to a hospital ship, and refine the radiation cure. She decides to stay behind to do this work and says farewell to the TARDIS crew, but the Black Guardian insists that Turlough must kill the Doctor now.
It's too bad that such a dark and powerful story was tripped up by technical difficulties: problems with costumes, electrical faults, and a badly built set. Much of it was improperly lit, production ran late, and scenes were hastily taped, making Peter Davison frustrated and angry. Nyssa lost her skirt in Part Two and wore only a slip in most of the story, ostensibly because she was ill and found the clothing restrictive, but she claimed she did it to please the many fans who complained that she was too covered up. Liza Goddard, who plays Kari, is the ex-wife of Sixth Doctor Colin Baker. The Vanir are from Norse mythology. Garm was the guard-dog of Hel. This was the end of the four-person TARDIS team until the 13th Doctor, Jodie Whittaker. Turlough was given Adric’s room. The Space-Time Element, or Heart of the Tardis as the Black Guardian calls it, is beneath the console. When the TARDIS crashes into the ship, it changes appearance and is indistinguishable from the corridor wall, which apparently means the Chameleon Circuit suddenly works.