Jace and Clary rush to make a final stand, but when a traitor is revealed, Jace and Clary may have a bigger fight on their hands than expected. Meanwhile, Alec and Izzy must fight off a large mass of demons that have unexpectedly shown up, as Luke and Simon head to the Seelie Court to deal with some unfinished business with the Seelie Queen.
After centuries of being lied to and treated poorly by The Clave, the Downworlders decide to come together and take matters into their own hands as the hunt for Valentine and Sebastian intensifies. But what starts off as a way to preserve their own, the Downworlders choice to break with the Shadowhunters, could quickly escalate to war.
After a devastating attack at The Institute, everyone is on high alert as the Shadowhunters close in on their hunt for Jonathan. Meanwhile, Simon and Maia help a new downworlder as the Seelie Queen demands an answer from Magnus and Luke.
With two of the three Mortal Instruments missing, everyone is on the hunt for the last one – The Mortal Mirror. Jace and Clary are on the hunt to recover it first before it could fall into enemy hands. But along the way, Clary realizes that her brother Jonathan may be closer than she thought. Meanwhile, Isabelle has a hard time accepting the fact that her little brother Max is growing up, and Maia struggles to bring her guard down with Simon.