I didn't spot any of the potential ongoing threads in tonight's Who. There was a vague suggestion towards the end of that "Doctor as hero/god/mythic figure" thing we've seen a couple of times this series, but it wasn't overt and was probably just me reading too much into things. There was lots of stuff about the Time Lords and the Time War, but that's been a theme since the show came back.
It was a cracking episode though. The underground city looked much better than a bunch of corridors with some junk strewn about had any right to, and the fish-men were really well done. The story was quite similar to last year's Gridlock, with an initially clichéd setting giving way to something considerably more interesting by the end of the episode. There wasn't much to Jenny (the titular Doctor's Daughter), but Georgia Moffett did well with what she had, and brought the character to life; I sense a spin-off, but I hope she'll be back in the main series to, if only to give the Doctor something to smile about.
It's the Agatha Christie episode next week, which is the one I've been most looking forward to for some reason.