[1] Emma Carstairs has been living with the Blackthorn family: her parabatai, Julian; his siblings Livvy, Ty, Dru, and Tavvy; and their uncle, Arthur, in the Los Angeles Institute since her parents' deaths during the Dark War five years ago.

According to Johnny Rook, a mundane shopkeeper from the Shadow Market, similar murders befell eleven faeries, whose kind are ostracized by the Clave under the Cold Peace for their support of the Endarkened.

Meanwhile, Julian struggles between running the Institute, since Arthur is functionally insane from his previous experiences in Faerie, and his love for Emma, despite parabatai being forbidden from pursuing a romantic relationship.

Diana Wrayburn occasionally helps the younger Shadowhunters with the upkeep of the Institute, but refuses to replace Arthur as leader and frequently leaves for other missions.

One day, the Wild Hunt, a group of traveling faeries led by Gwyn ap Nudd, arrive to deliver Mark, the eldest Blackthorn brother, who was kidnapped by them during the Dark War.

Changed by his time in Faerie, Mark finds solace only in Cristina Rosales, a Shadowhunter from the Mexico City Institute currently staying in Los Angeles.

With Mark's help, Emma finds a cave which serves as a convergence point for the ley lines, managing to take evidences before being attacked by demons.

Eventually, Ty completely translates the runes carved onto the victims' bodies and learns that the ritual, which requires thirteen sacrifices, also demands Blackthorn blood.

In the story, a woman was killed by her parents and her lover made a deal with the Faerie king to conduct a ritual which required Blackthorn blood and the Black Volume of the Dead.

Kieran visits the Institute and, in goodwill, reveals two pieces of information: Iarlath is working with Malcolm, and Emma's parents were killed to test the resurrection spell.

Some reviews like the on thenational.ae suggests good things about the book, it says, "Clare's imagination once again fills the pages with a thrilling world that promises political deviance, never-ending drama and unwavering love".

[4] The Palatinate.org.uk reviewed the book as, "Lady Midnight might not be Cassandra Clare's best work, but fans of The Mortal Instruments and The Infernal Devices will still likely find it an interesting read.