it's the Master! Oh my god! And the Doctor totally loses it and screams at him in the TARDIS and omg it's THE MASTER. SQUEEEEEEEEEEEEEEEEEEEEE

I actually, even though I knew the spoilers, I'm very slow to twig to things, so I didn't realize the Professor was him until halfway through? And it was when they were WORKING TOGETHER ON SKIENCE. Just like the old days!!!!!!