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AN: Hope everyone likes my explanation for Leela's survival; it's based on her final appearance in the "Gallifrey" miniseries, where Leela, Romana, K9, and their assorted allies were faced with the task of stopping an insane Time Lord from reaching Gallifrey in a TARDIS carrying several rapidly-decaying temporal weapons and a virus that would destroy the Time Lords' ability to regenerate
AN 2: The bit where Leela talks about instinct being more necessary than vision was inspired by the concept of slipstream piloting in "Andromeda", where slipstream- a sub-level of the universe that served as the Andromdea-verse's method of faster-than-light travel- could only be properly navigated by an organic pilot 'choosing' the correct paths (Even the series itself admitted that it was unclear why organic pilots were better at this than A.I.s, but that essential premise is the reason Leela was selected)
Old Faces, New Hopes
After a few moments in which the two old friends simply stood there sobbing- Martha and the Torchwood team having retreated to the conference room to give them some privacy-, Leela pulled back from the Doctor, her face still slightly wet from her tears as she looked him over.
"You have... regenerated?" she asked, smiling slightly at him.
"Twice since we last met," the Doctor replied with a nonchalant shrug even as he continued to grin at her. "It's... it's good to see you, Leela."
"You as well... Doctor," Leela replied, her gaze slightly wistful as she looked at him before another thought occurred to her. "Is... is Romana well?"
"Romana?" the Doctor asked, only just able to push his regret aside in favour of his curiosity; he'd known that Leela hadn't been involved in the events on Gallifrey from Romana's second regeneration onwards, but facts about the mess that Gallifrey had become during his time in the universe of anti-time had been rather hard to come by. "Leela... she's gone."
Leela blinked.
"Wh... what?" she whispered, her eyes wide as she looked at him. "But... but if you are here like this... the Free Time virus..."
"Free Time?" the Doctor repeated, looking uncertainly at his friend. "Hold on, I heard a bit about them... weren't they some kind of terrorist organisation that objected to us monopolising time travel?"
"Yes..." Leela replied, nodding at him. "Did Romana tell you... about the virus?"
"Weeeeell, more like I... picked up some bits as I went along; it was meant to make people susceptible to Free Time's influence but turned the infected into zombies, right?" the Doctor asked, unable to stop the sorrow on his face as Leela nodded. "Oh no... Leela, I am so sorry-"
"There was nothing... you could have done... to help us, Doctor," Leela said, shaking her head reassuringly at him as she held up her hand to halt him mid-sentence. "Even when I did not understand the specifics... of the trouble Romana faced, I understood... that it was not something you could have helped us with. This was a conflict of ideologies... of the old Gallifrey of Darkel and Pandora... against the Gallifrey Romana sought to create... and neither was wrong; you could have done nothing..."
"Doesn't mean I don't wish I could have been there..." the Doctor muttered, even as the brief smile of gratitude he gave his fourth incarnation's old companion made it clear that he was grateful for her reassurance.
He couldn't change what had been, but that still left them the future to deal with; the trials and failures of the past weren't worth brooding over at a time like this.
"So... what happened?" he asked at last, briefly racking his brain for the memory of what he'd read during those last few hours on Gallifrey before he continued. "According to the last records I found on the topic- things got a bit... complicated... when I got back; didn't have time to read the whole thing in detail-, there was an outbreak of the virus on Gallifrey?"
"There was," Leela replied, nodding in confirmation at him. "Romana, her K9 and I were trapped in Irving Braxiatel's old storerooms while a war-TARDIS carrying temporal weaponry and piloted by two recent victims of the virus was on a collision course towards Gallifrey, and the only direct means of time-travel that we possessed was a damaged Timescoop. Braxiatel and Romana were able to take parts from the Timescoop and combine them with K9 to allow him to travel in the time vortex to pursue the TARDIS, but a pilot was required for accurate control..."
"And you volunteered?" the Doctor finished for her.
"Piloting K9 in that fashion was described as needing instinct rather than vision... and Romana said that I possessed the greatest instinct of any she had ever known," Leela replied, a slight smile at the memory before she continued speaking. "With K9's senses already connected to the TARDIS we were tracking... I was able to successfully follow the temporal trail the ship had created to find it before it could reach Gallifrey. K9 was able to knock the TARDIS off-course from Gallifrey long enough for the temporal weapons on board to decay and destroy the ship while it was in orbit... but the resulting explosion separated me from K-9, and the next thing I recall is being awake on the Z'nai ship... with my vision restored-"
"Restored?" the Doctor repeated, holding up a hand as he looked urgently at her. "As in, you were blind at one point?"
"I had an... accident... involving a bomb detonation near a supply of artron energy; the doctors told me that my eyes were gone from me forever, and I still do not know what restored it," Leela replied, shrugging slightly as she looked at the Doctor. "And from there... well, that is all I really remember until you rescued me."
For a moment there was only silence as the two old friends looked at each other, neither of them willing to broach the topic that they both knew they had to acknowledge, until Leela finally spoke.
"What... happened, Doctor?" she asked, looking uncertainly at him. "The last time I saw you, you had exiled yourself to the realm of anti-time..."
"Well, long story short, I was purged of the Zagreus energies early on in my time there; came back to this universe after I learned what had happened to me and left the Zagreus parts of me back where they belonged," the Doctor said with a casual shrug.
"Ah," Leela said, before taking a breath to steady herself before she asked the main question. "And... Gallifrey?"
"Right..." the Doctor said, nodding thoughtfully for a moment before he smiled briefly at her. "Well, on an immediate note, I can tell you that Romana's plan to stop the TARDIS worked; Romana must have managed to recover K9 at some point after the two of you were separated, because last time I saw him he was living with a couple of friends of mine as my housewarming present to them-"
"What?" Leela interrupted, glaring at the Doctor as she processed what she'd just heard. "You gave him away? Why would you do that to Romana?"
For a moment, the Doctor could only sit in silence, his mind going over his possible responses- the truth might be the most obvious, but that didn't mean he had to like it-, before he pushed any thought of lying aside; after everything Leela had been through on Gallifrey, she deserved the truth.
"Well," he began, taking a deep breath as he looked at her, "firstly, you'll be pleased to know that what you did saved Gallifrey; the detonation of all those temporal weapons in a TARDIS's state of temporal grace triggered a wave of benevolent temporal energy that caused all biological life in the immediate vicinity to regress backwards a few months, healing your eyes and erasing any trace of the Free Time virus from the planet."
"Really?" Leela asked, looking at him slightly uncertainly. "It was... that simple?"
"Well, the conditions aren't exactly easy to duplicate- the odds of the TARDIS detonating at just the right distance from Gallifrey for the resulting temporal waves to have that effect without causing a dangerous level of cellular regression are rather slim; you could have easily caused a significant portion of the planet to regress to protoplasm-, but essentially, yep; you single-handedly saved Gallifrey and restored your sight in one moment," the Doctor said, smiling at her before the smile faded. "But... well, it didn't last."
"It... it didn't?" Leela asked, looking apprehensively at the Doctor. "What... what happened?"
"Daleks," the Doctor responded, his tone grim as he addressed his old companion (He had already made up his mind not to reveal the full extent of Romana's role in the destruction of Gallifrey; his old companion had done what she could at the last minute, and tarnishing her memory wouldn't accomplish anything). "They attacked Gallifrey, Leela... a single, massive strike, penetrating all our defences and shutting down everything we had available to us in a matter of moments; all the reconstruction Romana had done since the civil war in preparation for this assault was undone before they could even try to mount a counter-attack."
"Oh no..." Leela whispered in shock, her eyes wide as she stared at the Doctor.
"I tried everything," the Doctor said, his head bowed slightly as he spoke, unwilling to look in Leela's eyes as he revealed his most private shame. "I did all I could to find another way... I tried everything I could think of... but in the end, there was only one thing left to do."
"You..." Leela whispered, her horrified gaze still fixed on the Doctor.
"I destroyed Kasterborus," the Doctor stated simply as he looked up at her. "All of the energy within the TARDIS was released in one blast; the Dalek fleet, Gallifrey, Kasterborus... all gone in a single moment."
He lowered his head with that statement, tears slightly brimming in his eyes as he stared at the floor, unable to look at his old companion until Leela's hand on his shoulder prompted him to look up at her.
"You killed your own people," Leela said simply.
"I did," the Doctor replied; honesty was what Leela deserved at this time, so honesty was what she would get.
"You had no choice," she continued.
"I should have-" the Doctor began.
"You did not," Leela interrupted, her gaze fixed on his as she spoke with that same simple directness that had so defined her attitude in her time with the Doctor. "You are not a killer, Doctor, nor are you a warrior as I am; you are a thinker, a speaker, and a teacher before all else, striving to help your opponents see things your way and cease what they are doing before you must make them stop... and if you took such a terrible action, it would have only been because all other options that you might take were far more terrifying and impossible to accept."
She smiled slightly at the Doctor as he stood before her, reaching out to place a comforting hand on his shoulder.
"I forgive you, Doctor," she said simply.
For a moment, the former warrior of the Sevateem wasn't sure how to respond when the Doctor wrapped his arms around her and pressed his face against her shoulder, silently shaking with barely-audible but nevertheless-significant sobs.
The sound of footsteps nearby prompted Leela to look up, but relaxed when she saw that it was merely the dark-skinned woman in red leather she'd seen earlier; judging by the concern on the woman's face as she looked at the Doctor, Leela guessed that this woman was the Time Lord's latest companion...
Then the warmth and affection in the woman's eyes for the man in her arms struck Leela, and she couldn't stop herself from smiling slightly.
It looked as though the Doctor had found his own true mate at last...
She just hoped that things ended better for the Doctor and this woman than they had ended for she and Andred; after all that the Time Lord who had given her so much had been forced to sacrifice for this universe, losing the woman he loved after so long fleeing love would be a blow she doubted even he could truly recover from...
AN 3: For those who don't know, Andred was a Chancellory Guard on Gallifrey whom Leela fell in love with after he helped her and the Doctor thwart a Sontaran invasion of Gallifrey in "The Invasion of Time". Their relationship fell apart in the first season of the "Gallifrey" audios when Andred regenerated after a confrontation with an assassin for the Time Lords' Celestial Intervention Agency- attempting to infiltrate the Agency, Andred claimed that he was the Time Lord who'd tried to kill him after disintegrating the other man's body, not considering what his actions would do to Leela until it was too late due to post-regenerative trauma-, Andred later dying during a civil war on Gallifrey before he and Leela could effectively reconcile