"Oh for pity's sakes... THAT HASN'T HAPPENED YET!" she shouted, only because he had complained about shouting. "The Master got dragged back to Gallifrey and somehow must've made it out. Take who I was when I arrived here... now add Rassilon and his usual compassionate, nurturing personality. Is it any wonder I turned out a bigger mess than before? He might not have helped you, but he didn't kill you!"
Missy got up and started pacing back and forth in front of the Doctor, gesturing with her arms as she spoke. "We both know what a complete pain in the arse I can be. But you of all people should also know that he was changing since arriving here. If you truly thought that we were unredeemable, then you wouldn't have worked so hard to keep us here and alive. You would've let him burn through his life energy, or let him leave when he asked to."
She whirled to face him and pointed. "What were our odds of winning? When the Master and I left, what were the chances that your plan would actually succeed?"
Miss snapped her fingers and pointed at him again. "Wait... if the Master already experienced it at some point in his timeline, that means those memories are locked away inside of my mind somewhere. Fine... we'll try it."
She walked over and sat on his lap, placing her hands on either side if his head. "Do it. I don't care if it hurts, or how deep you have to push. But find out what happened. I won't stop you."