I've got to admit, I like a stand-alone episode to be exactly that, one you could take out or move around, so, for me, I didn't like the tick-tock rhyme at the end. I can see your point, with the arc they have in this series and a stand-alone story concerning a child, it was much harder to ignore the arc. I'm still excited for all the episodes though and want to see what Moffat does.