Terminus was the fourth story of Season 20 of Doctor Who. It was the second story in the Black Guardian trilogy. It saw Doctor Who’s take on the Big Bang and featured the departure of Nyssa.
Under the direction of the Black Guardian, Turlough manipulates the TARDIS’ circuitry in a corridor. Hearing Tegan approach, he hurriedly shuts the roundel he was working behind. Tegan, still suspicious of the new companion, finds him and questions him, her mistrust only exacerbated by the roundel falling open again. When Turlough tries to turn on the charm, Tegan shows him to his room, formerly Adric’s.
Once Tegan has left to find Nyssa and complain about his obnoxious personality, Turlough heads to the console room to remove the space-time element from beneath the console. The Black Guardian appears on the scanner screen to encourage his obedience, despite his doubts that the Guardian will preserve his life. While Turlough is unable to remove the element, his sabotage causes the TARDIS to begin breaking up.
Tegan sees one of the fractures in the corridor and runs to find the Doctor. Another fracture appears in Nyssa’s quarters. While the Doctor frantically tries to save the TARDIS, the fracture in Nyssa’s room resolves into a door with a mysterious skull logo on it. Deciding that the unknown on the other side of the door is better than the certain death in her room, the Doctor tells her to run through the door.
The Doctor, Tegan and Turlough run to Nyssa’s quarters. The Doctor throws a chair into the door to keep it from closing and severing the link to the space vessel that the TARDIS, engaging an emergency system, has locked onto. The trio crawl through the door to follow Nyssa, only to have it close behind Turlough.
The Doctor goes ahead to find Nyssa. Tegan and Turlough stay behind. Meanwhile, two space pirates, Kari and Olvir, board the ship and make their way to the bridge, finding it deserted and running on automatic. Hearing a sound she believes to be Nyssa, Tegan drags Turlough to find her. Instead, they find a locked door through which robed arms try to grab Tegan.
On the bridge, Kari and Olvir have taken the Doctor and Nyssa prisoner, believing them rival pirates. Unexpectedly, Kari and Olvir’s ship moves away. While Kari tries, first by force and then by negotiation, to persuade the Doctor to take them with him, a computer voice suddenly comes to life. It announces that they are soon to dock at Terminus and all passengers should disembark to avoid an imminent sterilization.
Olvir realizes what this means: the passengers locked in their rooms all have Lazar’s disease and they are trapped on a plague ship.
As the ship docks with Terminus, Turlough and Tegan take shelter under a grating in the floor of the corridor to escape the diseased Lazars. The Doctor warns Nyssa not to touch the Lazars, and stops Kari from shooting them. The three of them head to the bridge to find Olvir. Nyssa finds him cowering behind a chair.
While Tegan finds a ladder out of the underdecks, Turlough hears the voice of the Black Guardian warning him not to fail in assassinating the Doctor. On the bridge, Olvir explains that his sister died from Lazar’s disease. He tells them that Terminus is run by a private company, for profit, that uses fear of the disease to demonize the plague victims. While they claim to offer a cure at Terminus, he’s never heard of anyone coming back. The Doctor, meanwhile, is stunned to discover Terminus is in the center of the known universe.
Tegan and Turlough, still under the floor, see an armored man patrolling the deck. He gives an order: “Sterilize!”
Within Terminus, Bor is studying a display and muttering, “The reading’s still climbing.” Valgard, the armored man from the ship, enters and is horrified to see Bor crossing a line demarcated on the floor. He goes to follow, but stops at the line. Instead, he reports to his commander, Eirak, that Bor has entered the Forbidden Zone, only to be horrified at Eirak’s insensitivity and fatalism at their situation.
On the bridge, the computer warns that sterilization is imminent. Noxious gas flows through the underdecks where Tegan and Turlough are still trapped. The Doctor’s entourage on the bridge decide to try to find the entrance to the TARDIS. Nyssa begins to feel feverish, necessitating the removal of her skirt. Realizing that she’s infected, Olvir recoils, only to see her dragged away by a robotic drone.
Nyssa is brought to Valgard, who is complaining to Sigurd, another Vanir, about Eirak’s insensitivity. Referring to himself as a “baggage handler”, Valgard himself is insensitive to the plight of the Lazars, caring only for the Hydromel on board the ship. While Sigurd retrieves the Hydromel (stealing a vial to insert in his armor), Nyssa attempts to flee Valgard. Unknowingly, she gives Olvir a chance to slip aboard Terminus. Valgard takes her to meet the Garm. Shortly afterwards, the Doctor finds Nyssa’s skirt.
Eirak, resigned to Bor’s death, has Valgard call the Garm (a large humanoid with a dog-like head) to ensure that Bor’s armor is recovered. The Garm is summoned by a small call box and emerges from the Forbidden Zone.
Leaving Turlough and Tegan to escape sterilization, the Doctor and Kari flee to Terminus, unknowingly observed by Valgard. He knocks Kari out, then grabs the Doctor by the neck and begins to squeeze…
Kari regains consciousness and goes for her gun. Valgard uses the Doctor as a shield, so she ricochets the shot from her gun off some nearby scrap metal. The gun’s power pack drained, Kari and the Doctor flee into the Forbidden Zone.
While Olvir steals a Vanir helmet and robe to move around undetected, Tegan and Turlough finally escape from the crawlspace. Trapped with the Lazars, Nyssa appeals to the Vanir for help, to no avail.
Eirak discovers that some of the Hydromel is, in fact, colored water. Valgard bursts in, reporting of his encounter with the Doctor and Kari. He challenges Eirak’s leadership, and Eirak contentiously agrees that, should Valgard return from the Forbidden zone with the intruders captive, Eirak will step down as the leader.
While Nyssa presses one of the Lazars for information, Olvir is detected as an intruder, and Eirak leads a party to find him. However, they find a decoy Olvir had left, and Eirak believes there is no intruder. Seeing to the Lazars, Eirak decides that Nyssa will be the first to be treated.
Exploring the Forbidden Zone to find the source of the radiation they’ve detected since arriving, the Doctor and Kari come across Bor, now severely burned and in a fragile mental state, lugging a load of scrap metal. Bor leads them to the engines that drive Terminus, one of which is damaged and leaking radiation; he’s been piling metal in front of it.
Bor, struggling to focus despite his memory loss, warns the Doctor that if the damaged engine is to explode, it would threaten the entire universe. He also says that one engine has already exploded, a “long time ago”. Telling them to check the computer and follow the cables, his explanation is interrupted by Valgard.
Sure that the Doctor and Kari are spies from Terminus Incorporated, Valgard attacks the Doctor with his staff. The Doctor parries and knocks Valgard into the metal Bor had been stacking around the engine. The Garm arrives, and picks up the weakened Bor.
Meanwhile, Nyssa is chained up at the entrance to the Forbidden Zone, and Eirak summons the Garm to take her to her mysterious cure. The Garm, still carrying Bor, enters at the same time Olvir arrives to try to free Nyssa. Unable to free Nyssa, he instead tries to shoot the Garm, only to find his blaster completely ineffective. The Garm takes Nyssa away; Olvir follows them into the Forbidden Zone.
As the Doctor tries to follow the cables that Bor mentioned, Bor himself is being questioned by Sigurd. While still confused due to his radiation sickness, he mentions that the pilot of Terminus is dead, and that if he fires up the engines, the Big Bang will happen all over again. Eirak denies the dying Bor any Hydromel, thinking it to be a waste of their limited supply.
Turlough appeals to the Black Guardian for help getting back to the TARDIS. The angry Guardian at first punishes Turlough for his failure to kill the Doctor, then reluctantly tells him how to recreate the door to the TARDIS. He and Tegan, the latter of whom is still unaware of Turlough’s bargain, head back to the ship’s crawlspace.
As the Doctor and Kari reach Terminus Control and find the corpse of the pilot, Olvir discovers the Garm chaining Nyssa up in the engine room. He rushes to her, only to be caught off guard by Valgard. Turlough, meanwhile, has discovered the circuitry that the Black Guardian indicated; his manipulations of it result in a shower of sparks.
The Doctor discovers that Bor’s description was not accurate in saying a previous engine had exploded. In fact, Terminus was a time ship that had dumped its fuel while in time-flight to avoid an overload. The fuel, dumped into a void, had started a chain reaction, resulting in the biggest explosion ever: Event One.
Deducing what had happened, the Doctor comes to a terrifying conclusion: while the exploding fuel had created the universe, the same events occurring now would undoubtedly destroy it. He also makes a second discovery: that the fuel dump process was automatically started by the computer – a process that has just begun.
Valgard and Olvir continue to fight, while the Garm takes Nyssa away. The younger and healthier Olvir defeats Valgard, only to realize Nyssa is gone. Turlough and Tegan, meanwhile, see the door to the TARDIS begin to reappear, and return to the circuitry in the crawlspace.
Olvir attempts to retrieve his gun from the pool of radiation generated by the engine. Valgard comes to and warns him not to try, lest he receive a fatal dose of radiation. Valgard reveals that, like Olvir, he was once a pirate, and learned combat from Olvir’s commander. That commander later betrayed Valgard, turning him in for a reward and condemning him to a life of slavery on Terminus. Valgard begs Olvir for sympathy, but he ignores him to find Nyssa. After Olvir leaves, Valgard stands, revealing his ploy to be a ruse, and retrieves Olvir’s abandoned gun.
The Doctor and Kari are struggling to throw the switch to stop the fuel dump, but are unable to get it to budge. Meanwhile, Turlough and Tegan’s work in the crawlspace is interrupted by an announcement from the computer: the star liner is beginning to move. Tegan runs to the bridge to attempt to stop the departure. The computer ignores her protests as the engines begin to glow.
Turlough, alone in the ship’s corridor with the door back to the TARDIS fully materialized, smiles to himself. On the bridge, the computer announces the launch is being aborted. Turlough runs to the TARDIS console room.
Nyssa awakens in a white chamber to discover all the symptoms of Lazar’s disease are gone. In Terminus control, Kari watches the Doctor struggle with the control, and observes aloud that the pilot must have had the strength of a giant. The Doctor has a realization: the Garm!
In the console room, Turlough is again berated by the Black Guardian for the Doctor’s continued presence in the universe of the living. Turlough’s crystal begins to glow.
Olvir confronts the Garm about what he does to the Lazars. The suddenly loquacious Garm defends his actions as saving some that would otherwise die, and takes Olvir to Nyssa. Sigurd, meanwhile, decides to steal some Hydromel to ease Bor’s pain.
The Doctor and Kari summon the Garm. Olvir forces his way into the chamber in which Nyssa is recovering, and is rewarded for his efforts by Nyssa throwing him to the ground and straddling him. Nyssa has realized the the “cure” of radiation could work consistently if it were properly studied.
As the engine continues to build to overload, the Garm begins to force the override lever back. Terminus itself begins to shake. At first, even the Garm’s strength seems insignificant, but eventually he manages to push it back. After it is shut off, the Doctor disconnects the computer from the system. The Garm pleads for something in return: his freedom.
The Doctor smashes the box that controls him, and leaves to finish the shutdown of the engine. Valgard ambushes the Doctor and Kari, only for them to be rescued by Olvir and Nyssa. Nyssa believes that the Garm, now free, could help her improve the treatment of the Lazars. Valgard protests that, even if this were true, the Terminus Corporation wouldn’t care, and as long as they control the Hydromel, the Vanir are slaves. Nyssa, however, realizes she could synthesize and even improve the Hydromel. The Corporation would be unable to retaliate, since the soldiers they might send would never enter a Lazar colony. Like the Garm, the Vanir are now free.
Eirak returns, and Valgard produces the Doctor, reminding Eirak of their deal. Sigurd supports Valgard, and even Bor says, “It’s time for a little chat.”
As the Doctor advises Valgard on how to improve Terminus, Nyssa makes an announcement: she is staying at Terminus to help the Lazars. With a kiss on the Doctor’s cheek and a hug for Tegan, her life has changed.
Turlough slowly wakes up on the floor of the console room, as the voice of the Black Guardian echoes through the room: “This is your last chance, boy… Kill the Doctor!”