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.