Under instruction from the Black Guardian (Valentine Dyall), Turlough operates some blue 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 ship 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 ship that appears deserted aside from two space pirates, Kari (Liza Goddard) and Olvir (Dominic Guard), who take them captive. The ship’s motors suddenly start up as it begins a docking procedure. The computer announces via a tannoy voice (Martin Muncaster) that they are arriving at Terminus Inc. Skull-patterned doors open all over the ship and diseased humanoids emerge to shuffle 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. 

On Terminus, the Vanir are in charge of operations. They unload the diseased humanoids into holding cells and prepare to sterilise the ship in which they arrived. Nyssa is found by one of the service robots and, as she has contracted lazar’s disease, is placed in a cell along with the others. 

There is unrest among the Vanir. Eirak (Martin Potter), the leader, controls the supplies of the drug hydromel which they need to survive the effects of radiation. Valgard (Andrew Burt) and Sigurd (Tim Munro) are unhappy with the situation. The company seems intent on cutting back to the extent of making their working conditions intolerable. Another Vanir, Bor (Peter Benson), wanders off into the forbidden zone to try to locate the source of the rising radiation levels. The Garm (R. J. Bell), a huge dog-headed alien, 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 releases the Garm from the Vanir’s service by smashing the device with which they summon and control him. 

Nyssa, meanwhile, has been exposed to the radiation and left in a room to recover. She realises that the cure Terminus promises for lazar’s disease can actually be made to work. 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. 

Tegan finally emerges from the ship – Turlough has made his way back to the TARDIS without her – and meets up with the Doctor and Nyssa. Nyssa decides to stay on Terminus to help the Vanir perfect the cure for lazar disease, and they say their farewells. 

In the TARDIS, the Black Guardian gives Turlough a final ultimatum. He must kill the Doctor

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}