An episode of two halves this one. First of all we have to look at it as the conclusion to Part 1, and then we need to look at the whole story arc element that was introduced by the Master.
As the continuation / conclusion to the first episode I don't think it was a particularly satisfying one. It took an age to see the relevance of the Doctor's adventures in history, then the alien plan still didn't seem all that clear, and it all seemed rather rushed at the end, with Barton simply running away. Did the aliens intend to kill everyone or to take their places? If the former, couldn't they have just, like, killed everyone. Why rewrite DNA if just to leave billions of empty shells. Ada Lovelace's role was at least relevant, as a pioneer of computing, but Noor could have been any SOE operative, or a member of the French Resistance.
The companions didn't really have all that much to do after they had escaped from the most leisurely air crash in history. They just when on the run and hid out for a bit. They didn't really move the plot on that much - just biding their time until the Doctor had caught up with them. That escape from the crashing plane was a bit of a cheat. Why can't the Doctor just go back at the end of every adventure and leave a get out for herself and her companions?
One other thing I wasn't happy about was the Doctor's bald assertion that she is a pacifist. Talk about setting up some fairly dull conclusions to future episodes. Enemies are obviously going to be talked out of universal domination from now on, when what we want to see is a few of them getting blown up from time to time. It's one thing for the Doctor to abhor violence and use it only as a last resort, but to go wholesale down the pacifist line is going to lead to some bland storytelling.
The thing that this episode will be remembered for is the confrontation between the Doctor and the Master stop the Eiffel Tower, where he reveals that Gallifrey has been destroyed. This is confirmed when the Doctor returns home to see the Capitol in ruins. We then find out that it was the Master who was responsible, because of something he found out about the foundation of Time Lord civilisation. This brings up the Timeless Child reference from Series 11, and obviously hints at more than just the Doctor having something unknown about her background. A game changer of a sequence if ever there was one.
One last thing I should mention is the glaring continuity error from that Eiffel Tower sequence. The Master asked if he'd ever apologised for Jodrell Bank - which was clearly supposed to be a reference to Logopolis but that wasn't set at Jodrell Bank. It was supposed to be filmed there, but wasn't, although it was use for the special edition Bluray release of that story. Jodrell Bank is in Cheshire, but the ambulance that is seen in that story isn't a Cheshire one, so the Pharos Project wasn't based at Jodrell Bank.
Overall, a slight disappointment after the breathless opener, but made memorable by that shocking revelation.