Clockwork Princess

Clockwork Princess is the third and final novel in The Infernal Devices series by Cassandra Clare. It describes the adventures of Tessa Gray leading up to her final battle. Although the novel is written in third person, it is from the protagonist's perspective.

Aloysius Starkweather, the Shadowhunter head of the York Institute, oversees the Marking of his fearful granddaughter, Adele, for her first Rune in 1847. However, Aloysius realizes too late that she could not withstand it. In 1873, Charlotte Branwell, head of the London Institute, introduces Will Herondale to Jem Carstairs, a sickly Shadowhunter from the Shanghai Institute, the former quickly agreeing to study shooting with him.

In 1878, Tessa Gray is preparing her upcoming wedding with Jem when Gabriel Lightwood arrives to tell the London Institute about his father, Benedict's transformation into a demonic worm. Tessa, Will, Jem, Sophie Collins, Henry Branwell, Cecily Herondale, and Gabriel and Gideon Lightwood proceed to the Lightwood residence in Chiswick where Gabriel delivers the killing blow to his father. Will confers with Tessa and says that someone has bought the entire stock of yin fen in London. This makes Jem more and more weak.The two try to ask Magnus Bane for the medicine to no avail, though Magnus gives them a pendant that alerts to a demon's presence. The Institute receives a letter from Axel Mortmain, revealing the person behind the purchase as him, and offers them the yin fen in exchange for Tessa, which Jem vehemently opposes. Tessa and Jem agree to shift their wedding to the next day, the day when Jessie Lovelace is delivered back to the Institute after her trial. A carriage with automatons attacks the Institute, kidnaps Tessa, and mortally wounds Jessie, who talks about "Idris" to Will before she dies. Cecily realizes that what she said was that Mortmain is hiding in Cadair Idris, a mountain in Wales. Will goes to rescue Tessa alone, following Jem's tip after the latter learns of the former's love for her.

In the meantime, Consul Josiah Wayland is trying to prevent Charlotte from succeeding him by blackmailing Gideon and Gabriel to spy on Charlotte, but they confirm their loyalty to Charlotte. Aloysius, who interrogates a faerie for information about Adele, who was switched by the faeries, learns that Adele was Elizabeth Gray, Tessa's mother, meaning that Tessa is his half-Nephilim great-granddaughter. Wayland calls an emergency "meeting", a cover-up to prevent the Shadowhunters from assisting Charlotte to confront Mortmain. Aloysius, having tipped Charlotte to inform Tessa of his discovery, realizes that she was right just as automatons sent by Mortain attack the Clave and murder the Shadowhunters, including Aloysius and Wayland.