City of Glass is the third book in the urban fantasy series The Mortal Instruments by Cassandra Clare.

Clary is at Luke's house with Simon packing for her trip to Idris, where her goal is to save her mother.

Jace tries to persuade Simon to lie and say that Clary decided not to go to Idris, in order to protect her from the Clave, which he feels will try to turn her into a weapon.

When Clary visits the Institute, Magnus tells her that the Lightwoods had to travel through the portal, due to an attack by Forsaken.

Pulled through the portal, Simon wakes up in Alicante and realizes the truth behind Jace's motivation to not make Clary come to Idris.

When Simon arrives to leave for home, it is revealed as a trap to incriminate the Lightwoods so as to bring people back to the Clave's trust and he is imprisoned and questioned on his new ability to walk in daylight as a vampire.

Sebastian eventually joins Clary and drops her back to Amatis's home, who is disappointed at her actions.

At night, Jace goes to visit and save Simon, who refuses to escape, because it would shine suspicions on the Lightwoods.

However, Clary pulls back as she is not sure that Jace's feelings for her are not just a result of the demon and angel blood in their respective bodies.

Simon meets up with Isabelle to comfort her as Jace walks Clary back to Amatis's place.

When Clary gets downstairs to save Jace from his mistake, she reunites with her mother, Jocelyn, and reacts to this with anger.

During the decision process, Jocelyn informs Clary that Jace isn't actually her biological brother but, rather, Sebastian is.

[3] The intense plot, character development, and expanded worldbuilding make it a must-read for fans of urban fantasy and paranormal romance.