It's a return to the show for Toby Whithouse, who obviously hasn't had quite enough of writing vampire stories yet and brings them to Doctor Who as well. Despite the title and the Hammer-Horror look of the episode, with the backcombed brides of, er, Fish, this is a romp rather than a scary one (which is about right since we got the spooky stuff in the preceding two-parter.) And the episode delivers on the fun action and funny lines - the dialogue in particular feels a lot more characteristic of Whithouse than "School Reunion" did. Whether that's because the success of Being Human means he's trusted more than before and therefore rewritten less, or whether Moffat generally doesn't do as sweeping a rewrite as RTD used to, who knows? Full of lovely moments right from the start though, everything about the Doctor crashing Rory's stag do in the most unconventional way possible works, especially the surprise decision to use a comic beat to go into the pre-credits cliffhanger.
Arthur Darvill fits in well being given more to do and has good chemistry with the others (perhaps not surprising considering he and Matt Smith were already friends socially.) And not only does the very reason he brings him along show the Doctor having much more respect for Rory than he did for Mickey, but this treatment continues throughout. I don't think I'm going to end up on team Karen Gillan though, however hard I try. It's not that she's bad exactly, it's just I think there's meant to be layers to her personality she's not quite giving us. Take Amy's treatment of Rory, which is really schizo. I kinda feel like that should have been part of how she's not quite all there mentally, as has been intimated right from the start. Instead, when she was treating Rory very pointedly as an asexual being at the start it just makes her come across as an uberbitch.
Still, I really liked the episode overall, it was mainly spoiled only by things going a bit wobbly at the resolution, both story-wise and with the CGI (up until the dodgy effects near the end this was actually making me think once again how much this lower-budget series actually looks better than the preceding, more expensive ones.) Some of Whithouse's twists to the vampire story were great fun (the mirror explanation, the unconventional fangs which, once you find out they're fish creatures, you realise are modelled on piranha teeth) although their relationship to sunlight seemed a bit flaky (I get that it was direct sunlight only that killed, but considering how quickly it did for Francesco at the end the vampires were being rather cavalier about shielding themselves from it until then.)
But a hit overall for me, and while it's easy to be excited that Alex "Gilbert" Price is in the episode, as a swordfighting villain no less (and he's great,) Helen McCrory is just an amazing guest star. She seems born to wear those over-the-top outfits and you can't take your eyes off her whenever she's on screen. She's gone straight onto my list of favourite new-Who baddies, which isn't bad going considering she has to keep phasing into a fish/insect thing every so often.