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.

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.

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?

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.

Clary comes up with a plan that will bring Shadowhunters and Downworlders together, as Alec struggles for a way to help Magnus.

Jonathan begins his reign of vengeful terror as the Shadowhunters try to find a way to stop him. With only one hope, Clary must make a sacrifice that could have long-lasting implications for all. Meanwhile, wedding bells are in the air for one special couple.