With two of the Mortal Instruments now in Valentine's hands, the world of the Shadowhunters teeters on the brink of civil war. Jace and the Lightwoods are recalled to Idris, the Shadowhunters' remote and hidden home country, where a search has begun for the last of the Instruments, the Mortal Glass. Clary finds herself caught up in the chaos as the delicate social order of the Nephilim begins to shatter, pitting Downworlders and Shadowhunters against each other. When the City of Glass falls under attack, can Downworlders and Nephilim fight together to defeat Valentine, or will their longstanding hatred destroy them all? And when Clary discovers the unbelievable truth about her past, can she find Jace before it's too late?