This is either a link from the end of classic Who season 19 to the fantastic Big Finish audio episodes featuring Nyssa and the Doctor, or else a link to season 20, whose opening episode never mentioned the fact that they'd just accidentally (?) ditched a crewmate.
He had already disappeared under the console, settling in to replace the panels he’d opened during the last round of jury-rigging. Nyssa was still staring, dumbfounded, at the viewscreen where Heathrow Airport had been just a moment ago. To be sure, there had also been several looming security personnel and an Earth official working himself into a state of high dudgeon, but the Doctor’s precipitous departure had left behind more than just awkward questions.
“Doctor. What about Tegan?”
“Hm?” His voice was muffled. “Oh, she’ll be all right. Back at Heathrow at last, just as I promised.”
“Doctor, she’d changed her mind! She wanted to stay with us. You can’t simply abandon her like that.”
“Abandon? Nonsense. She’s been badgering me about that promise for weeks. Hand me the null spanner, would you?” His head popped out from under the edge of the console. He blinked at the rigid anger radiating from the petite young woman. “I’m sorry, Nyssa, but goodbyes can be so terribly awkward.”
“Don’t you ever go back for those you’ve left behind?” Tegan would have shouted it. Nyssa’s quiet, brittle tone was far more devastating. She deposited the tool next to him, then stepped over his legs and marched briskly through the inner door. Her heels clicked away into silence.
The Doctor raised his head to call after her, then puffed out his cheeks and fell back with a thump. He lay there staring at the ceiling and listening to the hypnotic hum of the flight deck. It felt and was a lifetime since the room had been so quiet. “It would be awfully crowded in here if I did,” he muttered. Which was an indictment in and of itself.
He finished sealing up the controls, then set off into the TARDIS' bowels to find her. This took some doing. The TARDIS had been discomfited by all the emergency pruning and rearranging they’d done to its layout during his recent messy regeneration. Since then, it seemed to be sprouting two corridors and three rooms for every one they had jettisoned. At least it had added a cricket pitch.
Some time later, more than an hour and probably less than a day, he finally located Nyssa in the new arboretum. On a high walkway among the treetops, the girl was tucked up on a bench with a thermos beside her and a large drawing tablet across her knees. Thrusting his hands in his pockets, he strode up with a tolerant smile. She did not turn. He leaned over her shoulder to inspect her work. The border of the page was tiled with meticulous sketches of flowers, leaves, and seed pods from the foliage around them. At some point, Nyssa must have gotten distracted, connecting many of the stems and leaves together with lines flowing into a complex electronics diagram.
Now she was gazing into space, meditating upon a nearby spiderweb. He wondered absently whether the TARDIS had spun it or brought aboard some real spiders.
The Doctor sat down beside her, leaned back on his hands and waited.
She pursed her lips and resumed drawing.
It was clear that he would have to make the first overtures. “Nyssa, I’m sorry. I realize that you and Tegan had grown close.”
“Yes.” She began to map out the spiderweb’s geometry with loose but precise lines, counting ratios under her breath as she traced the spiral.
“And parting is that much harder, when you've... lost a great deal in a short span.” A brutal metonymy, that, standing in for an entire world, for her father whose body now housed the mind of his murderer, and for Adric, whose deserted bedroom across the hall from hers must tug at her like a black hole. “You need something certain to hold onto.”
She blinked rapidly. It was the closest the Doctor had seen her come to tears. Were all children of Traken conditioned to such stoicism, or was that trait peculiar to her?
“My needs are not important.” She took a measured breath. “Adric… chose the manner of his departure. Tegan didn’t get that chance.”
“And so he is dead while she is safe.” The Doctor sounded suddenly very tired. “Which is best, Nyssa? What do you think I should have done?”
“It’s not for me to choose!” she said, eyes sparking.
“Tegan chose Earth,” he said patiently. “Do you really want to turn around and look for her, to ask if she’s quite sure? How do you propose we search for her, then? It’s a small planet, but it’s still rather a broad field to survey.”
Nyssa’s shoulders sagged. “Ah.” It was yet another loss, if not quite so absolute.
He gave her time to digest bald fact before continuing. “Look here. Nyssa, you do have choices. I can take you almost anywhere you wish, even Traken. Not quite as you remember it, but it was a peaceful, gentle planet for many thousands of years before your time. I believe I can trust your judgment not to tamper with history. Choose a moment, any point in the past, and I will take you there. I had hoped... selfishly, perhaps... to look after you for your father's sake. But in light of recent events, it might be better to install you somewhere out of harm's way."
“Dwelling in the past, quite literally.” Pensive gray eyes unfocused, turned inwards, sifting memories. “Tempting, but no thank you, Doctor. I have to go forward. I can’t go back.”
“Very well. The question then becomes: which way forward?”
“The question is rhetorical. From the TARDIS, every direction is forward.”
“Well, pick one.” He smiled. “Come now, Nyssa, is it so hard to choose?”
“Yes.” Unlike Tegan, her frankness was more often refreshing than abrasive. “I’m just not sure, Doctor. When I first joined you, I thought you yearned as I do, to set right whatever can be set right. But it seems as if wherever we go, things fall apart, and we can only repair half of what’s been broken.”
He nodded. “The Xeraphin. I gave them a way home, but only by trapping the Master there with them.”
“And that fire in old London: yes, it destroyed the mutated pathogen, but how do we know we didn’t set the whole city ablaze? And Castrovalva: it may have been a simulation, but can we be sure none of its inhabitants had a soul?”
He grimaced. “My track record has certainly not been stellar since you came aboard.”
Her expression softened. “I’m not blaming you, Doctor. The universe outside Traken is chaotic, dangerous, sometimes even hostile. I grasp the truth of that now. But I'm beginning to think it’s best not to meddle, lest we make things worse.”
“You’re too young to be second-guessing your ideals. I’m sure your father taught you about failure and persistence; I’ve seen how you tackle the TARDIS controls.”
She smiled faintly at that. “Well, if there weren’t so many malfunctions, I’d learn faster.”
“Malfunctions!” he said, drawing himself up with indignation. “Quirks are not malfunctions. The TARDIS is a unique machine, Nyssa, with circuitry as distinctive as a fingerprint. I’d expect you of all people to appreciate her mechanisms as art.”
"Then teach me." She raised her chin and one eyebrow, almost challenging.
“Is that really what you’d choose, then? Puttering about on an aged TARDIS, when I’ve offered you the entire universe to explore?”
"Doctor, I don't want to be just a passenger.”
“You know, that’s the most sensible thing I’ve heard from any of you since Romana left.” He slapped his knees and hopped to his feet. “Come along, pack up; we haven’t a moment to waste. No time like the present, eh?”
“You’d know.” She flipped the tablet closed and stood up. “So, where are we going?”
“The Heart of the TARDIS. It’s high time you were properly introduced.” He beamed. “I fancy you'll get along with her very well.”