This final installment follows Tessa and her friends as they face off against the series main antagonist, The Magister (also known as Axel Mortmain), who plans to completely obliterate the Shadowhunter race.

Two months after the events of the previous book, Tessa Gray is in the midst of preparing for her wedding to Jem Carstairs.

Jem gets sick faster and Will realizes that he has taken an unusually higher dosage of yin fen so he can stay stronger for the wedding.

However, Axel Mortmain has bought all of London's stock of yin fen, wanting to blackmail the Institute into handing over Tessa in exchange for the drug.

Meanwhile, Consul Josiah Wayland, now increasingly frustrated by Charlotte Branwell's refusal to be his lapdog, attempts to convince Gideon and Gabriel to betray her in exchange for the Lightwood's honor to be restored.

Back at the institute, Charlotte's attempt to rally the Enclave to support her fails, managing only to gather Sophie, Cecily, Gideon, Gabriel, Magnus Bane, and three Silent Brothers for the mission to stop Mortmain.

Meanwhile, Wayland gathers the Clave for an emergency meeting to depose Charlotte, only for the automatons to attack them and kill him and Aloysius Starkweather.

Upon arrival at Cadair Idris, Tessa is forced to shapeshift into John Shade and teach Mortmain how to create automatons capable of powered by demonic spirits and immune to Nephilim weapons.

Upon her recovery, Jem, now Brother Zachariah, visits Tessa and promises to meet her every year for an hour at Blackfriars Bridge before departing for Silent City.

Afterwards, Tessa leaves her family for good, unable to see any more of her loved ones dying, and becomes a wanderer, though she continues to keep in touch with Jem every year, while tracking her descendants from afar.

In 2008, more than seventy years after Will died, Tessa is shocked to see that Jem is no longer a Silent Brother, thanks to a twist of events.

Anne Zafian, VP and deputy publisher of Simon and Schuster Children's Publishing, states, "We are always trying to bring something new to the table while promoting Cassie's books...Since she has such a good online outreach and social networking skills, it was great that, with the Twitter campaign, the first chapter could be revealed worldwide at one time.

[9] Other marketing efforts for the novel included the release of Clare's Shadowhunters App for Apple and Android, which launched early February 2013 and gave a first look into the book's prologue.