"I know about the gate," the Doctor said, his expression darkening. "It's a Vinvocci medical device. The Master was able to use it to transmit his genetic template over the entire planet back in our own universe. He used it to bring the Time Lords back as well. Not that it helped him much. He was still dying. He used his wife to bring himself back from the dead, you see. Again. The thing is ... " The Doctor paused and turned his tea cup between his hands, considering. He looked lonely and a little lost, like a child whose Christmas hopes had been dashed.
"The thing its ... I wanted him to be different. Better. He saved my life after he realized what he'd unleashed with the Time Lords. I still don't know exactly why, if he just wanted to have his revenge on Rassilon, of if there were more to it. Romana told me he was different, and I wanted to believe her. Evidently, he's not different enough."