Alec, Jace and Isabelle make a plan to finally get Clary back and stop Jonathan—much to Luke’s and Simon’s chagrin. Meanwhile, an emotionally raw Magnus struggles to deal with his father’s reappearance and Maia must make a tough decision that could affect the rest of her life.
To try and save Clary from Jonathan, Jace goes undercover. Back at the Institute, Isabelle, Simon and Luke race to find a way to finally destroy Clary’s bond with Jonathan. With time running out, can Isabelle create the solution before Jace’s cover is blown?
A demon outbreak in New York City on Halloween becomes a top priority at the Institute, but as the Shadowhunters hunt the demons, Jace becomes more worried about Clary. Meanwhile, Alec has to juggle the outbreak with his deep concern about Magnus’ depression.
Clary and Jace come up with a risky plan to get rid of her twinning rune, which may bring an old enemy back into the fray. Meanwhile, Alec is worried about Magnus.
Continuing to track down information on the rumors about misconduct at The Clave’s downworlder prison, Isabelle asks Alec for help. Meanwhile, Magnus succumbs to asking Lorenzo for a favor and Jace takes Clary on a special date.
Simon is determined to get rid of the Mark of Cain and, with Isabelle’s help, tries to track down New York’s oldest vampire to get more information on the mark. Now in Paris, Clary desperately tries to find an escape from her current situation. Meanwhile, with his new sense of mortality, Magnus tries to live life to the very fullest.