It's eight years after Sebastian was defeated. Following a mysterious accident, an orphaned girl with a shady past is taken in by the New York institute. But this girl can't stay quiet forever, and the youngest Lightwood brother is determined find out what fatal secret she guards so carefully. Only, once he knows, there is no forgetting.

Cass comes from a long line of proud Gryffindors. She's the perfect child, always follows the rules, always polite. So when she's sorted into Slytherin, becomes best friends with Draco Malfoy, excels at potions and teams up with a poltergeist to get back at Harry Potter, her parents are more than a little a disappointed.