"You. Are. Not. ALONE!" To me, one of the best moments of the new series of Doctor Who. It still gives me chills to watch that scene. Derek Jacobi gives a chilling performance and I honestly felt sad that "Professor Yana" was only a disguise and he was going away. Plus I really loved how so many seeds for this story were planted so far in advance. I think I want to go watch it again now.
And as usual, the time lapse video.