A desperate journey begins as Clary Fray prepares to enter Alicante, the shimmering capital of Idris, the ancestral home of the Shadowhunters. Her mother, Jocelyn, lies in an enchanted coma, and only the warlock Ragnor Fell holds the key to waking her. Yet, entering the city without official permission is forbidden by the Clave, the Shadowhunters' governing body, and doing so could mean death. Jace, her newfound brother, and the Lightwood family are already in Idris, attending vital meetings with the Clave to discuss the looming threat of Valentine Morgenstern, Clary's nefarious father.
However, a sudden attack by Forsaken forces the Lightwoods and Jace through the portal to Idris in haste. Clary, determined to follow, creates her own portal, defying Jace's earlier attempts to keep her safe from the Clave's scrutiny. She finds herself in Lake Lyn, a poisoned lake, and though she survives, her arrival is met with Jace's cold displeasure. He believes her presence endangers them all, especially as her best friend, Simon, a newly turned vampire, is soon imprisoned by the suspicious Clave for his unusual ability to walk in sunlight.
Amidst the political intrigue and her own forbidden feelings for Jace, Clary seeks out Magnus Bane, the High Warlock of Brooklyn, who reveals that Ragnor Fell is dead. The only hope lies in a powerful spell book hidden within Valentine's old home. With Jace's reluctant help, they infiltrate the abandoned mansion, discovering a captive, withered angel, Ithuriel. Through visions granted by the angel, they witness Valentine's dark past: his experiments with demon blood on his unborn son, Jonathan, the very name Jace believed to be his own. This revelation shakes Jace to his core, leading him to believe his supposed demon blood is the cause of his forbidden attraction to Clary.
As Alicante itself comes under a devastating demon attack, its protective magic shattered, the Shadowhunters and Downworlders are forced to fight side-by-side against Valentine's monstrous horde. The city burns, and tragedy strikes the Lightwood family with the death of young Max. Jace, driven by rage and a desire for answers, tracks Sebastian Verlac, a mysterious Shadowhunter who had befriended Clary, to a cave. There, he uncovers the shocking truth: Sebastian is Valentine's true son, imbued with demon blood, while Jace is the son of Stephen Herondale, having been given angel blood by Valentine while in his mother's womb.
Valentine, now possessing the Mortal Cup and Mortal Sword, discovers that the true Mortal Glass is Lake Lyn itself. He plans to use all three Mortal Instruments to summon the angel Raziel and fulfill his twisted vision of purifying the Shadowhunter race. Clary, realizing his intent, confronts her father at the lake. Jace, wounded from his battle with Sebastian, arrives, only to be stabbed by Valentine with the Mortal Sword. Using Jace's blood, Valentine successfully summons Raziel. Yet, in a climactic twist, Clary, with her unique rune-creating abilities, alters the summoning ritual, causing Raziel to turn on Valentine, destroying him utterly.
With Valentine defeated, a new era dawns for the Shadowhunters and Downworlders. The Clave, witnessing their combined strength, agrees to a new alliance, allowing Downworlders a voice in their government. Jocelyn awakens from her coma, confessing her love for Luke, Clary's werewolf protector, and they begin to plan a future together. Clary and Jace, now free from the burden of their perceived siblinghood, can finally acknowledge their profound connection, their love no longer a mortal sin but a beacon of hope in a world forever changed.
