City of Glass by Cassandra Clare (Walker Books, 2009) is the third installment of the young adult urban fantasy Mortal Instruments series and chronicles an epic Shadowhunter battle against an old foe’s return. City of Glass was a finalist for the Teen Choice Book of the Year and for the 2009 Goodreads Choice Awards. Clare was born in Iran to American parents. She spent several years traveling before she settled in the United States, where she attended school and worked for several magazines, including The Hollywood Reporter. Before her debut novel, City of Bones, was published, she wrote Harry Potter and Lord of the Rings fanfiction and published it online. Currently, she resides in Massachusetts with her husband.

This guide follows the 2010 Margaret K. McElderry Books edition of City of Glass.

Plot Summary

City of Glass picks up one week after the end of City of Ashes, the previous installment in the series. Clary Fray prepares for her first visit to Idris, the Shadowhunter homeland, to search for a way to wake her mother from an enchanted sleep and defeat Valentine—her father and Shadowhunter-turned-villain who’s allied himself with a demon army. Valentine seeks the Mortal Glass, the third of the Mortal Instruments, to summon the angel Raziel and “purify” the Shadowhunter race, installing him as the new ruler of the Shadowhunters, able to kill anyone who doesn’t align with his standards. Jace, a Shadowhunter still grappling with his conflicted feelings about Valentine—whom he believes to be his father—joins Clary’s mission along with a growing group of young Shadowhunters who also question whether downworlders (beings that are part human and part demon such as werewolves or vampires) are actually friends and not enemies, as they’ve always been taught.

Fearful that the Clave—the Shadowhunters ruling body—will try to weaponize Clary’s latent Shadowhunter powers for their own purposes, Jace initially prevents Clary from going to Idris, which results in him accidentally bringing Simon (Clary’s vampire friend) into the country with him. After drinking Jace’s blood in City of Ashes, Simon has become a daylighter—a vampire able to walk in sunlight—and the Clave takes him prisoner, seeking information about his transformation, as well as a way to blame Jace’s family for Valentine’s return to power. Angry she was left behind, Clary portals to Alicante, the capital city of Idris, where she meets Sebastian, a Shadowhunter a few years older than her who seems vaguely familiar.

Clary learns her mom used a potion from an ancient spell book to put herself to sleep, and with Jace’s help, she infiltrates one of Valentine’s safe houses to find the spell book. They also find a bound and tortured angel, who shows them images of Valentine’s experiments with demon blood on his unborn son, Jonathan—the name Jace believes he was given at birth. Overwhelmed with shame, Jace is sure his demon blood is the reason he’s attracted to Clary even though she’s his sister, despite the fact that Clary, who has no demon blood, wants him too. Frustrated at a lack of answers, Jace and Clary return to Alicante, where demons ravage the city on Valentine’s orders.

Valentine gives the Shadowhunters a day and a half to make him their undisputed leader before he sends another wave of demons to attack. Clary and Jace free Simon from the Clave’s prison, where they also find Hodge Starkweather, Jace’s old mentor from the New York Shadowhunter Institute, who reveals that the Mortal Glass is not a mirror as they believed, but a lake. Sebastian overhears everything, reveals himself to be a spy for Valentine, attacks Jace and Clary, and escapes. Badly wounded, Jace and Clary return to Alicante, where they learn Sebastian has killed a Shadowhunter child, Alec and Isabelle’s little brother.

In the night, Jace slips away to track Sebastian. After he leaves, Clary and the others learn Sebastian is the real Jonathan, Valentine’s demon son. Meanwhile, the Clave decides to submit to Valentine’s demands. Knowing it’s the only way to change their minds, Clary reveals the power she has over runes, offering a new rune that will let Shadowhunters and downworlders share abilities in order to overpower Valentine’s forces—only by working together can they defeat him. The Clave reluctantly agrees.

Independent of one another, Jace and Clary each learn of Valentine’s plan to summon Raziel at the mirror lake. Jace fights and kills Sebastian and rushes to the lake, where he finds Clary bound and silenced. Valentine kills Jace and uses his blood to summon the angel, unaware that Clary has already disrupted the ritual by switching the name in the pentagon to her own so the angel won’t be bound to grant Valentine’s request. Valentine explains his desire to purify the Shadowhunter race, which the angel recognizes as a prejudiced power grab. The angel kills Valentine, and Clary uses her one request to bring Jace back to life.

With Valentine’s defeated, the demons battling the Shadowhunters disperse. After burying their dead, including Valentine, the Shadowhunters and downworlders throw a victory celebration, where Jace and Clary—now officially not siblings—finally confess their feelings for one another. Amid their friends, they snuggle close as fireworks explode overhead.