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.
With Valentine and Sebastian on the loose, the Downworlders come together to make a surprising choice.
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.
With everything that Simon has gone through this past year, a family dinner for Yom Kippur – the Jewish Day of Atonement – poses as a real challenge. How can he come clean about being a vampire and especially about what just happened to his family? And if he is stressed out, what if he can’t control his vampire urges? Looking to take care of this himself, Maia insists on helping him out whether he likes it or not. Meanwhile, Clary and Jace go on an unsanctioned mission and test out their newfound rune powers.
Simon goes down a dark path while Alec and team prepares to transport Valentine.