[Considering this is the fifth book in a series there are, needless to say, spoilers for the first four.] 
 So, we almost had a happy ending with City of Fallen Angels. But now, Jace is missing and the Clave decides it's not a priority anymore, because he's probably dead and, besides, they have other things to do. So who else would continue searching for him, besides our faithful pack of Shadowhunters and Downworlders? 
 Sebastian's back in town, and Jace is bound to him. Literally. He used some voodoo dark magic to bind them together, so you can't injure one without harming the other--which makes it really hard to kill Sebastian, as much as they want to. 
 I won't lie; I only really got into this about halfway through. Clary spends so much time being touristy with Jace in European cities, and it's like she forgets why she's there at all. It's not until Sebastian's evil plot is underway that you remember her purpose for being there--and that's when the action really starts. 
 There's a lot of forced relationship-type stuff in here. Magnus & Alec are less... Magnus & Alec. Alec, boy, why are you ruining a good thing? Jordan & Maia are supposedly back together, which... okay, I don't understand that. And now Simon & Isabelle have this thing? Listen, you can live a perfectly happy life without pairing everyone off. It's a bit excessive. 
 Let's just go back to battling demons and saving the world. It doesn't need all this smushy lovey-dovey filler.