I loved this episode because it reminded me of classic Doctor Who, it had the same formula (though usually those got several episodes to solve the issue, this one had to do it quick).
So for that classic Doctor Who feel, and formula, A+, this is what Doctor Who was, is and should be.
OH, and he mentioned his granddaughter Susan, not by name, but he said he'd been there before with his granddaughter
This brought a huge smile to my face, acknowledging that she existed, and now we know of one place they went to before Earth. Maybe one day he'll answer what actually happened to her after leaving her on Earth in the future with the Daleks.
Also, glad we are back to the Doctor and the Companion just being friends, and not lovers, never did like the new plot in Doctor Who of the companions falling in love with The Doctor, or vise versa.
And BTW, I loved the singing, the songs were great, and everyone did well, I'd buy that on a soundtrack.
Only complaint, too much use of the sonic screwdriver, it is literally the deus ex machina.