It all begins when Turlough,during another attempt to sabotage the TARDIS by the direction of the sinister Black Guardian,winds up disrupting the time circuits of the TARDIS to such a degree it ends up docking with the nearest available ship via a built in fail safe device. Aboard the doctor and Nyssa encounter a group of ill passengers and space pirates attempting to raid the ship while Tegan and Turlough become trapped in it's elaborate ventilation system. The ship turns out to be a transport vessel for Terminus,a giant space hospital complex located at the center of the universe. The people on board suffer from Lazurs Disease for which their is apparently no cure and which Nyssa begins experiencing symptoms of. Once the doctor and one of the pirates make it to the bridge of Terminus,they discover the vessel is the source of "event one" more often known as the big bang and,being as time/space capable as the TARDIS was at one point is about to start another catastrophic event.
For her part the infected Nyssa is taken in by the Vanir,guardsmen clad in radiation proof armor who possess the only possible treatment for Lazars. This is portioned out in small measures by a dog like creature called the Garm. Once the doctor encounters him,turns out the Garm is a benevolent but controlled entity who is able to deactivate Terminus's ailing engine in time to stop the catastrophe. By this time fully cured,Nyssa realizes it's the high radition levels caused by the engine that are the source of Lazurs disease. But the head of the Vanir named Eirak isn't about to let his own awareness of this go public on Terminus since a parent organization known as the Company are rationing supplies of the illness's one cure hydramel to the Vanir only. In the end Nyssa and the doctor are able to convince all the Vanir to begin producing hyrdamel themselves,in lieu of the company's stronghold over them. With this understanding as the doctor,Tegan and Turlough are set to depart back on the TARDIS Nyssa decides to put her scientific knowledge to good use to help reorganize the medical situation on Terminus. Once aboard the TARDIS,however Turlough is again menaced by the Black Guardian,threatening him if he doesn't carry out his mission to kill the doctor.
Within the story Nyssa's reasons for leaving the TARDIS to assist the ill on Terminus is not only laudable but,very much in character for her,is highly practical and logical. The story of Terminus is an excellent commentary on worldwide health care problems. Due to the face that the health care given on Terminus caters not to the care givers or patients on the ship but to the middle men that meddle the treatment,the Vanir have become highly corrupt and the patients left hopeless to fend and eventually die for themselves,with little real research to done on the illness itself. While Nyssa enjoyed the company of the doctor and her fellow companions,her scientific knowledge was would have vital impact aboard Terminus,whereas the doctor could do her job many times over on the TARDIS. Her goodbye is one of the most emotional and genuine since the first doctor said goodbye to his granddaughter back in 1964. The theme in the story of discord and mistrust is heightened by Turlough's self centered wish to return to his world. As well as his lack of willingness to kill for the Black Guardian in order to do so.