Terminus ( TV story ) Terminus was the fourth story of Season 20 of " Doctor Who " . It was the second story in the Black Guardian trilogy . It saw " < nowiki > Doctor Who ' < / nowiki > " s take on the Big Bang and featured the departure of Nyssa . Synopsis . The TARDIS attaches itself to a space liner after Turlough , still under the Black Guardian ' s influence , damages its controls . The Doctor and Nyssa meet two space pirate s , Kari and Olvir , who have come on board the liner in search of plunder , while Tegan and Turlough get lost in the infrastructure . The liner docks with what appears to be a hulk floating in space . This is Terminus , which claims to offer a cure for Lazar ' s disease . It is crewed by armoured slave workers , the Vanir . The cure is administered by a huge , dog - like creature known as the Garm . Nyssa , who has contracted the disease from sufferers transported aboard the liner , discovers that the cure - involving exposure to radiation - does actually work . The Doctor and Kari learn that the ship , once capable of time travel , was responsible for the creation of the universe when fuel ejected from one of its engines caused the ' big bang ' . Aided by Kari and the Garm , the Doctor disconnects an active but damaged engine that is on the point of exploding - something that could result in the universe ' s destruction . Plot . Part one . 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 Doctor goes through the door and tells Tegan and Turlough to stay . After a few minutes , Tegan goes through . The Black Gaurdian tells Turlough to go through and to kill the Doctor . Once Turlough gets through the door it closes behind him . The Doctor goes ahead to find Nyssa . Tegan and Turlough aren ' t sure which way the Doctor went . Nyssa , hearing someone coming up behind her , hides . It is the Doctor , though , and he finds her . 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 along to help find her . They find a locked door that the voice is coming from . Tegan tells Turlough to find a pry bar and works to try to open the door . While he ' s away , he hears her yell and comes back to find robed arms trying to pull her inside the door . He helps her to get free of the arms . On the bridge , Kari and Olvir have taken the Doctor and Nyssa prisoner , believing them rival pirates . Unexpectedly , Kari and Olvir ' s ship undocks and accelerates 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 sterilisation . Olvir realises what this means : the passengers locked in their rooms all have Lazar ' s disease and they are trapped on a plague ship . Part two . 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 demonise 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 centre of the known universe . Tegan and Turlough , still under the floor , see an armoured man patrolling the deck . He gives an order : " Sterilise ! " Within Terminus , Bor is studying a display and muttering , " The reading ' s still climbing . " Valgard , the armoured 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 sterilisation 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 . Realising 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 armour ) , 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 armour is recovered . The Garm is summoned by a small call box and emerges from the Forbidden Zone . Not knowing what happened to Turlough and Tegan , the Doctor and Kari flee to Terminus , unknowingly observed by Valgard . He knocks Kari down and then grabs the Doctor by the neck and begins to squeeze . . . Part three . 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 powerpack 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 , coloured 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 behind him . 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 . Part four . Valgard and Olvir continue to fight , while the Garm takes Nyssa away . The younger and healthier Olvir defeats Valgard , only to realise 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 by using his staff . 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 materialised , 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 realisation : 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 ready to strike . Nyssa has realised that 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 , realises she could synthesise 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 ! " Story notes . Production errors . " to be added " Home video and audio releases . DVD releases . This story , " Mawdryn Undead " and " Enlightenment ( TV story ) " were released in " The Black Guardian Trilogy " Boxset on 10 August 2009 . Contents : Box set . It was released as issue 140 of " Doctor Who DVD Files " .