With Simon being held captive at The Hotel DuMort, Clary pleads with Jace, Alec and Isabelle to help her rescue her best friend. As the team readies for the mission, Jace helps Clary learn more about her Shadowhunter powers, Isabelle turns to her favorite Seelie for Intel, and Alec begins to question Jace's motives and his place in their partnership.

Meanwhile, a very scared Simon gets very up close and personal with Camille, the leader of the vampire clan.

After getting a clue about who may have wiped Clary's memories, the crew hunts down the High Warlock of Brooklyn, Magnus Bane. With all of the warlocks in hiding from Valentine, the team must lure Magnus out with something he can't refuse - a killer party and something very valuable. But with emotions running high, more may come out of their meeting with Magnus than just the retrieval of Clary's memories.

Meanwhile, Simon doesn't seem to be himself after his recent Downworlder encounter.

After piecing together the clues as to where her mother hid the Mortal Cup, Clary, Jace, Alec and Isabelle are on the clock to find its hiding place before anyone else. But with demons and Downworlders lurking everywhere, obtaining the Cup may not be as easy as they had hoped.

Meanwhile, as Simon's symptoms become increasingly worse, he fears he is turning into a vampire.

With word of Alec, Jace and Isabelle’s “extracurricular activities” with Clary reaching The Clave, a convoy is sent to observe the Lightwoods running the New York Institute. Once The Clave representative arrives, it is very clear how much trouble the Lightwoods are in.

Meanwhile, Clary is faced with a devastating decision.

The Institute is in full swing as everyone is preparing for the wedding of Alec and Lydia. Lydia is getting final touches made on her wedding dress, the wedding decorations are set and Isabelle is trying to figure out a way to throw a traditional, mundane bachelor party for her brother – whether he likes it or not. During what should be a time of great celebration for many is far from it as they examine their relationships. Alec is struggling on two sides – his wedding is one of the biggest days in his life and his parabatai should be at his side, yet he and Jace are still at odds. Alec also continues to struggle with his feelings for Magnus, especially when Magnus won’t give up on Alec. And as the Institute is buzzing with romance, Clary is desperate to work things out with Jace, but he is keeping her at arm’s length. Meanwhile, Jace, Clary and Magnus go looking for warlock Ragnor Fell to try and find the antidote to awaken Jocelyn.

Jace, who is still reeling from everything that has happened, makes hunting down Valentine his number one priority even though he continues to wrestle with his conflicted feelings. In hopes of finding a way to stop Valentine before it is too late, Clary and Simon must find the key to unlocking Jocelyn's coma. But that key to unlocking Jocelyn’s spell may just undo the Downworlder peace treaty, Simon's relationship with Raphael, and the Vampire clan in the process.

Meanwhile, Alec must deal with the aftermath of his decision and what his choice means for his future.

Clary continues to struggle with the new Shadowhunter order as Jace is being held in The City of Bones with an unknown fate, and all suspicion in the Shadowhunter world continues to rest on Valentine’s daughter. When duty calls, Clary gets a little reprieve from the family drama as she helps Alec and Isabelle hunt down a powerful demon. But what starts out as a routine mission quickly puts the Institute on high alert, and what happens next will change everything for the Shadowhunters and their Downworlder friends. Meanwhile, Jocelyn has a proposition for Clary as Raphael and Simon ramp up their hunt for Camille.