It's the first of a two-parter concluding with Flesh and Stone and it stars River Song before she became terrible. And frankly, she's still good here and I'd rather see her powerwalking through spaceship interior for 45 minutes than seeing entire Angels lore being dismantled. So basically it's about how everything we knew about Angels is wrong. Remember how we shouldn't blink? Well, guess what, you are out of luck, fucker. And they won't send you back in time. And they can talk.