Sartiacaradinora, or Sartia for short, was a renegade Prydonian Time Lady who attended the Time Lord Academy on Gallifrey with Romana. During her days at the academy, she acted as Romana's friend, but behind her back, she and the other students would chastise her for being an overachiever and a tutor's pet.
Sartia also believed the Academy taught nothing but "boring theory."
After Romana left the Academy, Sartia discovered some ancient records regarding a planet called Fundrell, which the Time Lords had once gone out of their way to hide. Presuming that some sort of ancient treasure lay buried there, she stole a Time Ring and left Gallifrey. She also took a staser with her. After her departure from her homeworld, she joined a documentary crew bound for Fundrell headed up by Eamonn Orensky. They ended up crash-landing on the planet's surface, making contact with the natives and capturing footage of them while they waited for a rescue team to come and fetch them.
When the Fourth Doctor and Romana arrived on Fundrell, the Doctor immediately recognised Sartia as a fellow Time Lord. She was sent to fetch Romana, who was separated from the Doctor, from a nearby village, but their reunion was cut short when the locals decided to sacrifice them to their god, Fundrell's Daughter, a giant Sleek. Sartia killed Fundrell's daughter in self-defence using her staser as she and Romana fled back to the ship. On the ship, Romana and Sartia attempted to decrypt the ancient Gallifreyan text to figure out the nature of Fundrell. (AUDIO: The Skin of the Sleek)
After Sartia attempted to abandon Romana in the gelatinous sea that the planet is composed of, she discovered that Fundrell was located at a nexus of parallel timelines and the planet itself was engineered by the Time Lords to serve as a "plug" to stabilise these parallel timelines. The people of the planet were left there to perform rituals using an ancient Gallifreyan tome in order to maintain the seal that was placed on the planet, but over time the rituals were forgotten and devolved into future-reading rituals and the Time Lords themselves forgot about the planet. Upon making this discovery, Sartia killed the ship's pilot, Klick Chervain, after he refused to aid her in her plans, then came into possession of another Gallifreyan artefact, an orb which gave her the ability to directly control the parallel timelines.
Sartia manipulated the timelines to make Romana open the door; she showed timelines where the Doctor and she were under attack and one where the Doctor was injured. When she chose the right one, she took the key and opened the TARDIS to connect the orb to the ship. Sartia noted when Romana pointed out the two were not compatible, the TARDIS and the orb were both old and they should be preserved in museums they were intelligent enough to work together. She attempted to use this orb in conjunction with the Doctor's TARDIS to make herself a god, this proved to be dangerous, as an array of timelines were tangled. She became mad and tried to persuade Romana even if she had tried to abandon her in the gelatinous sea, to join her.
Sartia ranted and raged at Romana when she refused, saying that Romana had everything and she would always be in the Doctor's shadow.
In the end, Sartia was knocked out from a jolt of energy resulting from the Doctor and Blujaw Skaldson "rebooting" the planet. When Sartia came to, she attempted to shoot the Doctor and Romana, only to have her staser knocked away by one of Orensky's flying cameras. She then proceeded to use her Time Ring to escape.