As she listened to his explanation, Missy could see the pain and disappointment in his eyes when he said that she had left him to go with her other self. "This vault... how long was I in there? As for leaving with myself, well... yes. It does sound like something that I'd do. But then again, maybe I had some kind of elaborate plan and only went with him to try and convince him to help you. I don't know. I don't have any of those memories, so I can't speak to what happened after we left."
Missy looked down at their hands and sighed. "If I had stayed, it wouldn't have been to help them. It would've been to help you, like I did with Rassilon. It sounds like what you were trying to do with the me in the vault is what you were trying to do with the Master after he arrived."