18-year-old Clary Fray finds out that she comes from a long line of Shadowhunters - human-angel hybrids who hunt down demons. When her mother, Jocelyn, is kidnapped, Clary is thrown into the world of demon hunting with mysterious Shadowhunter Jace and her best friend, Simon.

The Shadow World is in upheaval as Valentine and The Circle continue to grow stronger by the minute, while the Shadowhunter team quickly becomes disillusioned with The Clave and their reaction to the recent events. Loyalties will be tested, unlikely bonds will be made and relationships will be pushed to the limit as the Shadowhunters and their Downworlder friends make their way in a new world.

While the Shadowhunters deal with changes in their personal lives and Clary and Jace keep dark secrets, a serious threat to the Shadowhunters emerges in the form of Lilith, a powerful demon building an army of disciples on a quest to resurrect her "precious boy" Jonathan.