A very enjoyable story I thought. David Bradley gave a terrific performance as William Hartnell. I can well remember when I was a kid we all ran around the playground pretending to be daleks. A nice touch where Hartnell is contemplating the end of his doctor and looks across the Tardis console and Matt Smith is there and smiles back at him. Reminiscent of the Endeavour pilot when he's asked where he thinks he'll be 25 years from now and looks into the rear view mirror and we saw John Thaw's eyes looking back at him. A touching and loving nod across the ages from the present incumbents to the original creator's and actors of both series I think.
A very well written script from Mark Gatiss also, he deserves a lot of praise for AAISAT in my opinion. A terrific tribute for The Doctor's 50th anniversary in my opinion.