Series 20: Terminus

Under the Black Guardian's instructions, Turlough sabotages the TARDIS, causing parts of it to dissolve. As the field of instability threatens to engulf Nyssa's room, a door appears behind her and the Fifth Doctor tells her to go through it. The TARDIS, to save itself, has materialized aboard a spaceship heading for an unknown destination. The Doctor and Nyssa, while exploring the ship, encounter two raiders, Kari and Olvir, who are intent on plundering the ship's cargo.

When the raiders' ship abandons Kari and Olvir, it becomes apparent that the spaceship is actually a transport carrying lazars, sufferers of a leprosy-like disease, to a space station named Terminus. The station is owned by Terminus, Inc., which claims that a cure exists there, but no-one has returned from it. Nyssa, separated from the Doctor, is infected by Lazar's Disease and ushered away with the rest of the lazars. Terminus is manned by the Vanir, guards clad in ornate radiation armor. They are slave labour, kept alive only by regular doses of a drug called hydromel, which is supplied by the corporation.

The Doctor discovers that Terminus is at the centre of the known universe and finds this information unsettling. Nyssa, meanwhile, is given over to the Garm, a giant dog-like biped, who takes her to a chamber and exposes her to radiation. The Doctor and Kari find the control room of Terminus and he realizes that Terminus is also a time ship. In some unspecified past, the fuel that powered Terminus became unstable and the now dead pilot had tried to jettison it while still in the time vortex. The tank exploded, and the outrush of energy started Event One - the Big Bang - and hurled Terminus billions of years in the future. There is still one tank of unstable fuel left, and the computer has begun a countdown to jettison that too. However, where the first explosion created the universe, the second will undoubtedly destroy it.

Nyssa awakes to find out that she is no longer infected. The radiation cure works, but it is haphazard, with as many people dying from it as recovering. The Garm knows this, but is unable to refine it as he is controlled by the Vanir. Enlisting the Garm's help, the Doctor staves off the countdown long enough to disable the computer and cut the engine control wires. In return, the Doctor destroys the electronic control box, setting the Garm free.

Nyssa strikes a bargain with the Vanir - in exchange for synthesizing hydromel and freeing them from the corporation's influence, they will turn Terminus from a leper colony into a true hospital, and with the Garm's help refine the radiation cure. Deciding that her scientific skills are needed more on Terminus, Nyssa elects to stay behind, bidding her friends a tearful farewell. As Tegan and the Doctor return to the TARDIS, the Black Guardian tells Turlough that this is his last chance to kill the Doctor...

Cast
Doctor Peter Davison (Fifth Doctor)
Companions Sarah Sutton (Nyssa)
Janet Fielding (Tegan Jovanka)
Mark Strickson (Vislor Turlough)
Guest stars
Valentine Dyall — Black Guardian
Liza Goddard — Kari
Dominic Guard — Olvir
Andrew Burt — Valgard
Tim Munro — Sigurd
Martin Potter — Eirak
Peter Benson — Bor
R.J. Bell — The Garm
Martin Muncaster — Tannoy Voice
Rachel Weaver — Inga

Production
Writer Stephen Gallagher
Director Mary Ridge
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6G
Series Season 20
Length 4 episodes, 25 minutes each
Originally broadcast February 15, 1983 – February 23, 1983

source: wikipedia