Meanwhile, the Doctor is doing the most awful thing this Doctor does, which is tell everybody to shut up while he's thinking, in a way we're meant to think is charming but is really just misanthropic and awful. Smith does the job of undermining the shittiness here, but the shittiness of it is real. The Doctor talks his way through the paragraph above -- while being adorable and snuggle-physical with Rory and Amy -- and realizes that she's going to sink Venice. And then repopulate it with her Veela brood mares, by giving them to the ten thousand husbands. Finally this offends somebody.
The Veelas attack, and there's a shitload of running around, but you already know what's going to happen because of the gunpowder in Act Two. So while Isabella's Dad sacrifices himself nobly by blowing all the Daughters of the Water to hell, obviously, the rest of the episode is happening.
Rosanna is boiling the skies over Venice and the Doctor is getting bossy with Amy -- "We don't discuss this! I tell you to do something, Amy, and you do it!" -- about her personal safety and getting big ups from Rory for joining Team Paternalist Patronizing. Which, Companion is a dangerous job. She signed up for it. Several times. You can't say, "Come out into the dangerous world" and then five minutes later be like, "Don't be endangered!" It's completely dumb, and it gets pulled out at the most random fucking times, just like as a way to get around plot points. But Amy's a person, not a plot point, so it's grody. On the other hand, her look of total betrayal and sadness is more effective than me whining about it, regardless of the Boys' Club deal that happens next.
Rosanna is just on fire today: "You're too late. Such determination, just to save one city. Hard to believe it's the same man that let an entire race turn to cinders and ash. Now you can watch as my people take their new kingdom." He tells her about the dead Veelas, and he's really sad for her, and he asks her to help him save Venice since the whole plan is now shot to hell. She's grateful for his sympathy, but also is now the last person of an extinct race, which is -- as usual -- thanks to the Doctor. Well, the Doctor and the Crack. Because her plan obviously had to be stopped. But still, she's got a lot going on.
Um, there's a swordfight between Rory and Frenchy. It is very basic stuff.
The Doctor reunites with Amy and Rory, who are all about not leaving his side once again, and yells at them about how "One minute it's You make people a danger to themselves, the next it's, We're not leaving you! But if one of you gets squashed, or blown up, or eaten..." Then the Doctor throws down some major science about the throne-weather machine and how there's another "hub" somewhere else and whatever, and he climbs up on a thing and does a thing, and there's lightning, but then the weather goes away again and everybody claps. Not for the Doctor, but because of sunshine.