The Mistress (The Choice)

"The Mistress" was a mysterious, long-lived humanoid woman. Stranded in the pocket universe known as Ecto-Space with K9 or B.E.S., she became one of its most celebrated heroes.

Origins
By the time of her encounter with Emperor Lukor, the Mistress described herself as 487 years old. (AUDIO: The Choice) Even her close associate B.E.S. considered her "something of an enigma", but wrote in her journal that according to "legend", the Mistress came from "an order of demiurges" who had "mastery over her home universe". She had gone by many names, including Eris, Freyja, Zorya and Danu, but B.E.S. did not believe that any of them were her real name. (AUDIO: B.E.S. Begins)

Travels with the Dominie
At one point, the Mistress travelled with a man whom she trusted absolutely. She would later state that "I was ready to give up my life for him. I thought it would last forever. That we would go on forever, as we were". Her travels with him and K9 apparently involved a lot of "recky". (AUDIO: The Choice) The Dominie mentioned to Tris'Tean that he had once known the Mistress, possibly in a slightly different timeline than the version of her who'd visited Jokastarna. He remained vague about the nature of their association, but carried a silver locket containing an image of a younger Mistress.

The Mistress recalled a trip her and "her mentor" had taken to Imperial Russia in the 19th century, intent on learning ballet dancing from Charles Didelot. Their lessons were interrupted by the need of thwarting interdimensional beings known as the Iele who wished to take over the universe, but she still managed to pick up the skill from Didelot's lessons. (PROSE: The Choice)

She eventually left him; unable to return to her original home due to the way her travels with him had changed her, she felt she had no choice but to move on to Ecto-Space in the company of a K9 unit. While delirious due to a fever, she seemingly repeated words from their separation: "But that's just it. Time to choose. I have to be my own--". (AUDIO: The Choice) These words echoed those of the Time Lady Romana when she split up with the Fourth Doctor and moved to E-Space with K9 Mark II: "But it is, isn't it? A moment to choose. I've got to be my own Romana". (TV: Warriors' Gate)

Hero of Ecto-Space
The Mistress partially rebuilt K9, giving him new abilities like solar power as a secondary power source. The two spent years travelling across Ecto-Space, eliminating slavery and helping populations deal with various plagues and diseases. On Ganamar, K9 suffered a temporary power failure and, in the face of a disease, the Mistress ended up going "old-school", using a mortar and pestle to brew up remedies with which she saved "at least 150 people". More famously, on Magronite 4 in the "beleaguered" Hanamar system, she was able to harness the locals' experiments with cypherdurite alloys to create an antidote to a disease that was menacing them. However, this put her at odds with the warlord Kebek. (AUDIO: The Choice)

Timeline fork
Two different timelines of the Mistress's travels in Ecto-Space existed. One version of her parted ways with her original, canine companion. She travelled alone for some time before being saved by a ship's A.I. after she'd been captured by murderous alien cultists. In gratitude, she saved the A.I. after the ship crashed, uploading her into the body of a robotic ferret. Dubbed B.E.S., this new robotic friend became the Mistress's new companion through adventures similar to the one the version of the Mistress who'd stayed with her original companion was having with him. On one occasion, the Mistress who travelled with K9 and the Mistress who travelled with B.E.S. even met one another due to a space-time anomaly. (AUDIO: B.E.S. Begins)

As Lukor's guest
Eventually, having become a celebrated hero across Ecto-Space, the Mistress took refuge from the wrathful Kebek's "thugs" at the court of Emperor Lukor of the Seven Systems, on the planet Jokastarna. She spent several months there as an honoured guest. These events occurred both in the timeline where she travelled with K9, (AUDIO: The Choice) and the one where she travelled with B.E.S.. (PROSE: The Choice) At any rate, with her robotic companion's help, she began to build a ship, which she hoped would allow her to travel both in time and in space, although she initially did not manage to get the time travel facility working. When Lukor abruptly proposed to her, she panicked and tried to escape in the unfinished ship, which merely flung her and K9 a few miles out from the palace, on the barren surface of the planet.

There, she discovered a network of caves under and around the palace, inhabited by the "caveys", blind humans with no knowledge of the outside world who worshipped Lukor's ancestor, the Empress Helena, as a near-deity, and had been suffering from a deadly plague for twenty generations, with only a few members each generation, called Chosen Ones, surviving. This briefly tricked her and K9 into believing they had gone back in time to the days of Helena's reign, which K9 had learned from the Court Historian saw much of the population being wiped out by such a plague. While trying to make her way to the palace, the Mistress was infected with the plague and captured by guards from above, come to collect caveys to carry out experiments on them.

K9 rescued her and managed to create a permanent antidote to the plague to save her life. They realised, as they made their way back to the palace, that they had not actually time-travelled, and that Lukor had set up the entire underground society, letting the caveys believe they were living in Helena's era, so that his scientists could infect them with artificial new strains of the plague for purposes of medical research — as Lukor was eaten away with paranoia about the plague that had killed his illustrious ancestor Helena having a resurgence.

After sneaking back into the palace, the Mistress and K9 acted out a charade with K9 claiming that he had used the last of his power and the remains of the ship to transport the two of them "back" to the present before succumbing due to having sacrificed all his power at once. The Mistress remained bedridden for a week and then claimed not to remember the circumstances of her return. She accepted Lukor's offer of a marriage, but, at the ceremony, interrupted her vows to instead expose Lukor's obscene experiments to the entire court.

However, even as he was beset by the angry crowd, Lukor warned her that she would be unable to return to her "beloved caveys": because his scientists had been able to extract K9's perfect antidote from her bloodstream, the experiment had been deemed no longer necessary. Making their way back to the Inner Kingdom, the Mistress and K9 discovered that the caveys had been systematically exterminated by means of irradiated gas. Overcome with guilt at not having been quick enough to stop Lukor before it came to this, she broke down crying before leaving the planet behind in the repaired ship. (AUDIO: The Choice)

On the Great Globe
Seeking to repair their ship's time-travel capability, the Mistress and K9 found a derelict Dyson sphere called the Great Globe, where they crossed paths with a group of archeologists. (AUDIO: The Search)

Behind the scenes
The character, played by Lalla Ward, was intended and heavily implied to be Romana II travelling with K9 in E-Space following the events of Warriors' Gate. Legally, she could not be named as such, as BBV Productions did not have license to the character of Romana. As documented in Downtime – The Lost Years of Doctor Who, the matter of the alternative name that would be used for the character was discussed during Bill Baggs and John Ainsworth's initial casting meeting with Lalla Ward at her home in Oxford, also in the presence of her then-husband Richard Dawkins. The name of "the Princess" was considered, but discarded because Dawkins "didn't think that would be a good idea, because princess was an American Jewish name for someone's daughter". Thus, "the Mistress" was eventually chosen.

Despite the legal impossibility of making the link explicit, The Choice still contains multiple allusions to the character's intended identity as an older version of Romana II. The Mistress mentions being several centuries old, and, while delirious from the plague, she babbles: "But that's just it. Time to choose. I have to be my own--". This is a near-quote of some of Romana II's last words to the Fourth Doctor as she chose to remain in E-Space in Warriors' Gate: "But it is, isn't it? A moment to choose. I've got to be my own Romana". She also wails: "You gave me no choice — I couldn't go back there, not after all this… Why did you do it to me?", seemingly addressing the Doctor, bemoaning the way her adventures with him made it impossible for her ever to return to Gallifrey.

When claiming to Lukor that she has been betrayed before as part of her deception, she again seemingly references the Doctor, explaining: "I trusted someone. I was ready to give up my life for him. I thought it would last forever. That we would go on forever, as we were. He betrayed me". This notably implies Romana/the Mistress had romantic feelings for the Doctor, something not made explicit on television except in the infamous Prime Computer advertisements, although it does, of course, come in a context of the Mistress presenting a disingenuous version of events to Lukor to trick him into thinking she loves him, which she does not.