It took a while to find him again after Galifrey and the Cybermen but she knew that a little thing like certain death wasn't enough to stop him after all it'd never stopped him before. The rage was new this level of rage at least, always before there had been a reason behind his actions be it boredom, Timelord induced madness or just wanting to get her attention. Flirting across space and time to paraphrase Missy. But this was insanity, to kill off their whole race, no his whole race if the memories of her origins could be believed. Even knowing that a part of him came from her surely that couldn't drive him to these lengths, not with everything they once were to each other.
She started to pursue him, following the rumours of him through space and time, travelling alone as it was far too dangerous for her fam to come along. It took time to track him from place to place his penchant for disguise serving him well. Everywhere she went he had not long left leaving whole planets in disarray though with surprisingly few casualties like he was being careful not to kill everyone in sight. She was getting more and more frustrated with each planet she reached just moments too late the lights in the Tardis flaring red and blue more and more often as the ship reacted to her friends anger and distress. He was laughing at her she could tell, leaving short notes written large in the destruction he left in his wake. His laughter echoing in the desolation.
She found him finally on an azure beach bounding a silver ocean on a lifeless world in the far reaches of empty space. She'd followed his footprints from his Tardis down to the waters edge, his back to her as he gazed out onto the calming waves. She stood beside him then joining him in silent appreciation of the view, watching the waves glisten in the sunlight. There was no need for words as she opened her mind and reached for his, the contact as he opened the connection stealing a small gasp of recognition from her. They'd spent so much of their childhood in each others minds that it had been odd and uncomfortable for them to be alone with their thoughts. They'd spent decades being friends, though that had been far too tame a word for what they were to each other. Even centuries of separation and enmity couldn't dull the familiarity and comfort she felt in having his consciousness beside her own. The careful walls protecting the vital systems and hidden secrets aside it felt good to know she wasn't alone even if it was the man who had ensured she was that was with her.
Soft as a whisper she asked him the question that had plagued her since learning of the timeless child and her own hidden history 'Why'. 'Why did you do it' she turned to him now watching his profile.
'Because of your name' came the reply, a murmur over the chasm between their understanding. He turned to her then seeing the furrow between her brows as she tried to grasp his meaning. 'Theta Sigma' he spoke aloud before continuing silently 'the two symbols that make up your name have another meaning. Test Subject.' With those words he lowered the walls around his mind sharing his memories of finding out what the Timelords had done to her, the rage he felt when he saw they hadn't even deigned to grant her a name and the horror of knowing that his entire race had been complicit in her subjugation. She watched as he raged on her behalf, as he tore down the citadel and rained hell upon his people for making a mockery of themselves and him.
Tears stained his cheeks as he shared himself with her his soul laid bare. Her eyes were vacant as she played the memories again and again trying desperately to find some reason some small evidence that it wasn't true that she hadn't been used to further the people of Galifrey before having the truth hidden from her for millennia. Bad enough that her childhood had been a lie, bad enough that there seemed to be no natural end to her existence, but even her very name was a painful truth buried in a lie.
Her own walls came crashing down at the realisation that nothing she was or had been was true. She was on her knees now the Master holding her as they both sank to the sand. Their minds mingling as she sought comfort from the one who had avenged her. 'Their gone now the can never hurt you again' he murmured into her hair as she clutched him close, weeping for the child she could barely remember.
Neither knew how long they stayed there holding each other as their hearts broke and then started to heal, the sun set and twin moons rose yet they stayed. Dawn was just breaking when they stirred pulling away and standing slowly still clasping arms as shaky legs took their weight. Together the walked back along the beach towards their Tardis', the Masters hand wrapped around the Doctors smaller fingers mirroring the intertwining of their minds.