After her medical work on Terminus, Nyssa is now the controller of a hospital ship, the Traken. As the universe burns in the crossfire of the Time War, she and her assistant travel to a planet close to Gallifrey where they are needed more than ever. A long time ago, Nyssa knew a Time Lord and understood his people. But it seems they can change...
The Traken is staffed by robot nurses called Sisters and sub-sisters. They look like columns.
Reave is strategically close to Gallifrey and is a natural source of praxis gas, which has a clean smell like a forest. The world's plants ease the reaction of praxis poisoning because they metabolise the gas, which turns them purple, creating an antitoxin.
Even though the Eighth Doctor is in the story, he travels incognito and is never directly referred to as "the Doctor" throughout the story. However, Nyssa recalls travelling with "her Doctor" and in the story the Doctor identifies himself as being three regenerations after travelling with Nyssa.