Had he just... The Doctor's eyes widened and he moved back until he hit the wall, staring at the Master with something akin to terror. He'd just called him Theta. Oh gods, what had he done? No, no, no... He tried to brace himself, but something about seeing Koschei's expression on the Master's face made that entirely impossible. He remembered the Master accusing him of wanting the past back and in a weird mix of horror and relief he realized that at least that wasn't true. This wasn't right and he knew it.

"...something went wrong. Ma..." He broke off and gave a quick shake of his head. "I'm sorry. I... I can fix it. Promise."