Turlough is still working with the Black Guardian to kill the Doctor, though at this point reluctantly. He's discovered the Doctor is only evil to the Black Guardia. Because of this, Turlough finds it increasingly difficult to carry out his task, he's not certain he can do it. So when the Black Guardian instructs him to sabotage the Tardis, he takes this more indirect route with the Black Guardians' promise he will be 'lifted away' to safety when the Tardis breaks up.
Tegan notices the Tardis is beginning to break up so calls for the Tardis, but Nyssa is trapped in her room. As a fail-safe, the Tardis locks onto a nearby ship and opens a door into the ship so she can escape. Using the Tardis scanner, the Doctor notices the door open in her room and encourages her to go through, knowing she'll be safer where ever she than in her room which is starting to dissolve. Once safe, the Doctor concentrates on the Tardis and fixes it. He then heads to Nyssa's room to see if he can find where she went.
Meanwhile, the ship is also boarded by space pirates who are uncertain what they will find inside. They quickly run into the Doctor who has been reunited with Nyssa, when they discover to their horror they are on Terminus, a leper ship where people go to either be cured or killed. Though told to stay behind, Tegan and Turlough follow the Doctor but are quickly trapped inside the ventilation system when they hide from a sentry robot. After ducking into the shaft, a group of Lazars walk over the grate leading into the shaft, and quickly becomes stuck.
As the Doctor, Nyssa and the two pirates are exploring the ship, Nyssa becomes very hot and takes off the skirt she's wearing. She's also not feeling well and realizes she's come down with the Lazar disease. Space Pirate Olvir tells Nyssa he had a sister with Lazar and she never came back from Terminus. It's not long before Nyssa is captured and brought to the slaves running the ship. The only thing that keeps them going is Hydromel, a drug that keeps them from getting the Lazar disease. If they don't work, they no longer receive shipments of the Hydromel. They are known as the Vanir and quickly process Nyssa for the 'cure'.
As the Doctor continues his search, unaware of Nyssa's capture, he runs into Bor, one of the Vanir who tells the Doctor a second of the ships engines is going to explode, as one already has. Having already known the ship is at the center of the universe and once capable of Time Travel, he realizes the ships first engine explosion is what started 'Event One'. If a second engine blows, it'll destroy the universe. The Doctor is about to help Bor, but is then captured by another of the Vanir who thinks The Doctor and Kari are corporate spies for the company that enslaves them.
The Doctor escapes and prevents the ships engine from exploding. Meanwhile, Nyssa is doused with radiation which does cure her, but to her horror she learns the process is unrefined and typical chances of survival are hit or miss. With the Doctors aid, she convinces the Vanir they can turn Terminus into a working facility, and she can process synthetic hydromel, thus freeing them from the company. The Vanir agree to help, so Nyssa opts to stay behind to help with continual research into the Lazar cure. She's confident the cure can be refined with a vastly higher survival rate.
DEPARTURE LOCATION: Terminus, a ship at the center of the universe.
REASON FOR DEPARTURE: Having been cured from the Lazar disease, Nyssa decides to spend the rest of her life finding a cure for other infected Lazars.