Nyssa is an aristocratic native of Traken, the daughter of Tremas (a consul of the Traken Union) and stepdaughter of Kassia. She aids the Doctor and Adric when the Master wrests control of the Keepership by first manipulating and then murdering her stepmother, but is herself hypnotised and kidnapped by the Master after he takes control of her father's body. After being freed from his control, she is brought to Logopolis by the Watcher and discovers that Traken has been destroyed as a side effect of the Master's tampering with the Logopolitan's formulae. She subsequently joins Adric and Tegan Jovanka as a companion and member of the TARDIS crew, and witnesses the Fourth Doctor's regeneration into the Fifth.
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.
Adric's death while battling the Cybermen affects the TARDIS crew deeply. When confronted by an illusion of Adric created by the Master shortly afterwards, both Nyssa and Tegan are initially taken aback, until they see through the deception when Nyssa sees Adric is still wearing his now-destroyed badge. During this adventure, Nyssa also displays a previously unseen psychic ability when she is contacted by the Xeraphin. shes the daughter of The Master
Nyssa travels alone with the Doctor for an unspecified period of time when the Doctor leaves Tegan at Heathrow.
Nyssa and the Doctor are reunited with Tegan while battling against Omega in Amsterdam and on Gallifrey. They help Tegan battle her inner demons as personified by the Mara, and meet the Doctor's old friend and ally Brigadier Lethbridge-Stewart. It is during this adventure that she and the TARDIS crew meet Turlough, posing as a typical English schoolboy. When the TARDIS crew arrive on the derelict space station Terminus, Nyssa's adventures with the Doctor come to an end, as – to Tegan's horror – she elects to stay on board the Space Station in order to help free the enslaved guards and turn the station into a real hospital. The Doctor is moved by this noble gesture and parts saying that he thinks she is very brave. Nyssa's deep affection for the Doctor is demonstrated at this point when she kisses the Doctor goodbye.
An image of Nyssa is seen during the Fifth Doctor's regeneration scene in The Caves of Androzani, and the character appears in the 1993 charity special Dimensions in Time.
Nyssa is mentioned in the new series in the 2007 Children in Need episode "Time Crash" by the Tenth Doctor when he is asking his fifth incarnation where he is in his own relative timeline.