I don't think a summary is possible, as there was genuinely so much going on, a fast paced, dark, action packed, witty script, excellent special effects.
The penultimate episode of Series 10 was superb, perhaps the darkest episode since Dark Water. I'd be so bold as to say this is perhaps the best episode I've seen for some years, I can't remember feeling so desperate for a concluding episode since perhaps Journey's End. Some truly chilling moments, Bill walking round hooked up to a drip, with early Cybermen sat around.
Incredibly good performances, John Simm returned with a bang, a fabulous performance from him, not the out of control Master we last saw, but a measured, controlled performance, and nice to see him back in a disguise. The interplay between him and Gomez was a highlight.
I loved the almost 60's music that accompanied it, took me back to The Tenth Planet, the big win though came from the return of the Mondassian Cybermen, about time, they worked incredibly well. They looked and sounded just as I'd wished they would.
My only criticism here will be aimed at the media, I purposely ignore news on the show, avoid the trailers, but far too much was given away by the media. I can only imagine the impact of Mr Razor's unmasking, had I not seen the revelation in a newspaper.
Unbelievably good. 10/10