Shadowhunters: The Mortal Instruments season 1 episode 6
Episode title: Of Men and Angels
With Luke in dire need of help, Jace, Clary and Simon take him to Magnus' lair in hopes of saving his life. Once there, the story of how Valentine came to be evil, and Jocelyn and Luke's past is revealed to Clary in hopes of triggering her memory of what happened to The Mortal Cup. Meanwhile, Isabelle and Alec are tasked to restore their family name in the eyes of The Clave.
