Tegan Distrusts Turlough

Under instruction from The Black Guardian, Vislor Turlough, who has just joined The Doctor and Tegan Jovanka in the TARDIS, operates some switches hidden behind a roundel in a corridor of the TARDIS. He then goes to the Console Room and the Guardian tells him to rip out the ‘heart’ of the TARDIS. He tries to remove the piece of equipment indicated, but it gets stuck. A wall of shimmering light starts to encroach upon the interior of the ship, trapping Nyssa.

Hurrying to the Console Room, The Doctor notes that the time rotor is jamming and activates the cut-out. By refocusing the scanner monitor on the interior of the TARDIS he is able to see and speak to Nyssa. A door emblazoned with a skull pattern appears in the wall of Nyssa’s room. The Time Lord tells her to go through it, and she does so. The Doctor, Tegan and Turlough then rush to her room. The strange door starts to close, but The Doctor jams a chair in the gap. He goes after Nyssa. Turlough and Tegan also pass through the door, which closes behind them and then vanishes.

The Doctor finds Nyssa and together they locate a control room. They are on a spaceship that appears deserted aside from two space pirates, Kari and Olvir, who are intent on plundering the ship's cargo. As the space pirates take them captive the spaceship’s motors suddenly start up as it begins a docking procedure. The computer announces via a tannoy voice that they are arriving at Terminus Inc…

The Black Guardian

Skull-patterned doors then open all over the spaceship and diseased humanoids emerge shuffling to the exits. Olvir realises that they are on a plague ship. To avoid the crush of people, Tegan and Turlough open a hatch in the floor and hide in the air vent beneath. When the coast seems clear they try to emerge but find that they cannot reopen the hatch. They decide to go along the venting instead. The Doctor meanwhile discovers that Terminus is at the exact centre of the known universe.

Terminus claims to offer a cure for Lazar Disease and is crewed by a group of armoured slave workers, the Vanir, while the cure - which involves the sufferer being exposed to a high dosage of radiation - is administered by a huge dog-like creature known as the Garm. On Terminus, the Vanir unload the diseased humanoids into holding cells and prepare to sterilise the spaceship in which they arrived. Nyssa is found by one of the service robots and, as she has contracted the disease from sufferers transported aboard the liner, is placed in a cell along with the others.

Exploring Terminus

There is unrest among the Vanir. Eirak, the leader, controls the supplies of the drug Hydromel which they need to survive the effects of radiation. Valgard and Sigurd are unhappy with the situation. The company seems intent on cutting back to the extent of making their working conditions intolerable. Another Vanir, Bor, wanders off into the forbidden zone to try to locate the source of the rising radiation levels. The Garm is summoned from the zone by the others and told to locate Bor and bring him back. The Doctor and Kari also enter the zone, and Eirak offers Valgard a deal: if Valgard brings back the two strangers, who are assumed to be from the company, Eirak will stand down in his favour. Valgard agrees.

The Doctor and Kari find Bor. He is trying to block the radiation, which is coming from a damaged engine. He is delirious and says that he followed the control cables. The Doctor and Kari do likewise and find the long-dead pilot of Terminus. The place was once capable of time travel, and The Doctor postulates that an ejection of fuel from one damaged engine could have created the universe. The second engine is also unstable and will soon explode, bringing the universe to an end.

Tegan and Turlough are still trapped in the conduits. The Black Guardian directs Turlough to what he says is an emergency bypass switch. As Turlough pulls out the wires on the switch, Terminus begins the procedure to jettison the fuel. The Doctor cannot prevent the jettison lever from moving and so recruits the Garm to help. The creature manages to push the lever back and prevent a catastrophe. By way of thanks, The Doctor destroys the electronic control box, which the Vanir summon and control the Garm, so setting him free.

Eirak

Nyssa, meanwhile, has been exposed to the radiation and left in a room to recover. Nyssa eventually awakes to find out that she is no longer infected. She realises that the cure Terminus promises for Lazar’s Disease can actually be made to work 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 has been controlled by the Vanir.

The Doctor and Kari return with Valgard, and Eirak is deposed peacefully. Nyssa explains that she can synthesise Hydromel and free the Vanir from the company’s control. It is then that Tegan finally emerges from the spaceship - Turlough has made his way back to the TARDIS without her - and meets up with The Doctor and Nyssa.

Nyssa then announces that she has realised that her scientific skills are needed on Terminus and so has decided to remain with the Vanir to perfect a cure for Lazar Disease and to help run Terminus as a proper hospital instead of a leper colony.

After bidding her friends a tearful farewell The Doctor and Tegan return to the TARDIS unaware that The Black Guardian is giving Turlough a final ultimatum. This is his last chance to kill The Doctor.