Alright, I feel like I should mention this and get it out of the way: there is not a ton of romantic action in The Eternity Cure. I'm assuming that we're aware of the fact that the action (and adventure) aspects of this series are what makes it so fun to read, but I'm tossing that out there just in case. 
 There's a lot going on in The Eternity Cure despite the deceptively simple sounding plot. There's Sarren seeking the labs for some reason, Kanin who Allison is trying to find (and save), Zeke who has been quite busy, and I'm going to stop myself right there. Definitely not the extent of The Eternity Cure, but enough to give you an idea of what's going on. You can expect to meet two different vampire Princes, see more of the group of humans living in the sewers, run into a ton of familiar faces, and the ending? Well, expect to flail (at least a little). 
 Honestly, that's probably my favorite part of The Eternity Cure, getting to see more of Allison's world. You'll also learn more about certain character relationships and character motivations. Also, while Allison does develop (as a character) somewhat, I feel like it took quite a while. We won't get me started on Stick (who I would like to beat over the head with a stick). Trust me, you'll see. 
 Overall, if you've read The Immortal Rules and need the extra push to pick up The Eternity Cure, I'm giving you that push right now. It didn't really feel--to me--like it suffered from middle book syndrome.