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.

Clary and Jace get a tip from Meliorn that they can find Valentine by locating a portal in another dimension, but only Clary can venture into this new world. What Clary finds in this alternative universe is that life there is as if the threat of demons no longer existed and Shadowhunters were no longer needed. While she realizes that this dimension is the life she always dreamed of, Clary must act fast or she will lose herself in the alternative world forever.

Meanwhile, Alec and Isabelle must face the consequences of the Downworlder attack.

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.