As with pretty much every six-parter, there's at least one episode that does little beyond padding out the run time...and "The End of Tomorrow" is pretty much a textbook stall. It's not a bad stall; the disappearance of Hartnell forces them to give David a bit more to do, which is nice as he's only got a couple more episodes left to become a romantic lead convincing enough to make us believe that Susan would want to stay with him instead of going off with her grandfather. They don't give Susan any of the Doctor's material, of course, but at this point we're all pretty much resigned to seeing Susan reduced to a portable scream.
But really, it's just an exercise in treading water. Ian and Larry wander around the labor camp narrowly avoiding Daleks and Robomen and stock footage; Barbara and Jenny very slowly work their way out of London; Susan and David wander around the sewers. None of it actually moves the plot forward more than a tiny fraction of an inch, but there's lots of incident to make up for the lack of actual events.
To be fair, one thing does happen. Susan and David bump into Tyler, who is quite frankly the most interesting and competent character onscreen right now. Actually, that's not fair to Jenny...she's terrific, and I find myself wishing she could have been a companion. Her over-protestation at the futility of Dortmun's pointless self-sacrifice is absolutely wonderful. Nation's supporting characters give me a lot more faith in his writing abilities than his regulars; Ashton is deliciously and unrepentantly amoral, and even Wells has realistic motivations and sincere heroism behind his trading with the black marketeer. There's a lot of depth to them, and Nation uses the extra time to develop them much better than supporting characters in Doctor Who usually get.
Other than that, there's not much to say about an episode with not much to say. The Dalek plan is still on "simmer", everyone's heading to Bedfordshire to meet Ian but they can't get there too soon, and Nation's run out of people to kill for now (although Ashton bites it, a bit too soon to be honest). Really, all that's left to comment on is the delightful notion of the Black Dalek's pet Slyther...I find myself wanting to know if he enters it in pet shows with Helen A's Stigorax. Do all the Daleks have Slythers? Are they all the rage back on Skaro, now that the fad for Varga plants has passed? Do they cuddle on the Daleks' laps, and get tickles under the chin with the suction cup? I know. I'm overthinking it. But in an episode like this, there's not much else to think about.