Nyssa is a alien from the planet Traken, the daughter of Tremas (a consul of the Traken Union) and stepdaughter of Kassia. She helps the Doctor and Adric to defeat Master. During her journeys with Tegan and Adric aboard the TARDIS, Nyssa finds herself trapped in a mathematical equation by the Master – whom she hates as he is, of course, now using "that face" (her father's). She also encounters a race of androids and their insane ruler, helps foil the genocidal plans of a wounded Terileptil and incidentally start the Great Fire of London, and discovers her remarkable resemblance to Ann Talbot.