Spoilers, spoilers, everywhere! (...for the previous books. I don't suggest reading this review if you plan to engulf yourself in this series. Which you should do.) 
 Tessa's engaged, but we know there's still angst brewing somewhere in the Institute. Will is pining over his parabatai's fiancee, but of course won't say anything. There are newcomers to the Institute, the Lightwood boys (Lightworm, if you ask Will), who throw everything for a loop. Oh, and let's not forget that Mortmain is still trying to get Tessa to complete his evil plot, and he's not above blackmail. Dirty, filthy blackmail. 
 But Mortmain's nasty letters go unanswered and he kidnaps Tessa, anyway. The last thing you want to do is piss of Jem & Will, and Mortmain is about to find out why. 
 You go, parabatai boys. 
 I can't say much else without spoiling things. But this is a wonderful ending to a heart-wrenching trilogy. I cried many times--tears of victory, sorrow, and joy. When the Shadowhunters finally go up against Mortmain, they're facing the impossible. When Will runs off to find Tessa because his parabatai can't take the trip, it's not known roped into and never want to leave. But all things must end. 
 And Jem. Oh, Jem; you may top my list of fictional boyfriends. I adore him so.