Content Warning: This guide describes and discusses the source text’s treatment of alcohol, substance, and gambling addictions.

In April 1878, William (Will) Herondale and James (Jem) Carstairs fight off a demon in an alleyway in London. Will and Jem are Nephilim, descendants of the angel Raziel, working as Shadowhunters, protectors of the supernatural world fighting against demonic forces. The two teenagers find a young girl’s corpse. She has been stabbed by a thin blade decorated with a double ouroboros. They assume the girl’s death to be the work of a Downworlder, a supernatural creature of the night such as a vampire, werewolf, or warlock. 

Meanwhile, Theresa (Tessa) Gray arrives in Southampton after her two-week voyage from New York City aboard the steamship Main. She expects her brother Nathaniel (Nate) to meet her at the dock and for the two of them to start a new life together in London. Instead, she is greeted by a coachman with strange, bulging eyes and two women called Mrs. Dark and Mrs. Black, who claim to be her brother’s associates. Tessa is initially suspicious, but the women have a letter from Nate that proves their claims, so Tessa agrees to take their carriage.