Tessa Gray should be happy - aren't all brides happy? Yet as she prepares for her wedding, a net of shadows begins to tighten around the Shadowhunters of the London Institute. A new demon appears, one linked by blood and secrecy to Mortmain, the man who plans to use his army of pitiless automatons, the Infernal Devices, to destroy the Shadowhunters. Mortmain needs only on last item to complete his plan.
He needs Tessa.
Charlotte Branwell, head of the Institute, is desparate to find Mortmain before he strikes. And Jem and Will, the boys who lay equeal claim to Tessa's heart, will doanything to save her. For though Tessa and Jem are engaged, Will is still in love with her.
The last words of a dying Shadowhunter provide the clue that might lead Tessa and her friends to Mortmain. But their small group cannot stand alone, and the powerful Consul doubts that Mortmain is coming. Deserted by allies, the Shadowhunters find themselves trapped when Mortmain seizes the medicine that keeps Jem alive. With his best friend at death's door, Will must risk everything to save the girl they both love.
To buy Will time, the warlock Magnus Bane joins with Henry Branwell to create a device that could help defeat Mortmain. As they all work to save Tessa and the future of the Shadowhunters that resides with her, Tessa realizes that the only person who can save her is herself - for in the discovery of her own true nature, Tessa could become more powerful than she ever dreamed possible. But can a lone girl, even one who can command the power of angels, face down an entire army?
Danger and betrayal, secrets and enchantment, and the tangled threads of love and loss intertwine as the Shadowhunters are pushed to the very brink of destruction in the breathtaking conclusion to the Infernal Devices trilogy.