The threat of Mortmain's Infernal Devices looms as the Shadowhunters race to work out a way to counteract the mechanical monsters. Meanwhile, plans for Tessa's marriage to Jem are underway, despite her fiancé's failing health and the unresolved feelings between her and Will. When Tessa is captured, Will must leave his comrade behind to save the woman they both love. But with the final missing piece-Tessa herself-in Mortmain's possession, can anything stop his plot for revenge?