The following version of the book was used to create this study guide: Clare, Cassandra, The Lost Book of the White. Simon and Schuster, New York, NY, 2020. Kindle AZW file.

Alec is a Shadowhunter, a member of an elite group of angel-blessed fighters charged with keeping the mundane (human) world safe from demons and Downworlders (including warlocks, vampires, werewolves, and faeries). Though the rules of the Shadowhunters forbids it, Alec is in a committed relationship with Magnus, a warlock who has lived some four centuries. As this novel opens, Alec and Magnus have possession of a magic book, the Book of the White. Though the book has the potential to be incredibly dangerous, Magnus has simply left it among the picture books in his son's room. When two powerful warlocks, Shinyun and Ragnor, manage to break through the wards of their apartment and momentarily hold onto Max, Magnus manages to retrieve Max but the warlocks escape with the book and leave Magnus with a magical hole in his chest. He learns the wound was created with the Svefnthorn, and that he will be bound to Sammael, a Prince of Hell, if he is stabbed three times.

Leaving Max with Alec's mother, Magnus and Alec prepare to track Shinyun and Ragnor to Shanghai, China, and several other Shadowhunters agree to travel with them. They include Alec's parabatai Jace, Alec's sister Isabelle, Jace's girlfriend Clary, Clary's parabatai Simon (who is Isabelle's boyfriend). They meet up with some Shadowhunters in Shanghai, including a young man named Tian who is involved with a young Downworlder. Though those relationships are technically forbidden, Tian is hopeful he will not always have to hide his love, citing Alec's and Magnus's relationship as a sign things are changing.

Sammael has been trapped out of the human world for centuries, but he is working his way toward world domination. Ragnor and Shinyun helped him reach Diyu, a realm of hell that has been out of commission since the former ruler there died, more than a century ago. When Shinyun opens a Portal between Diyu and Shanghai that allows demons to flow through, Magnus and the Shadowhunters leap through the opening in the hope of stopping Sammael. Simon is immediately taken from the group and tortured. They have trouble making it through the bureaucracy of Diya and Isabelle winds up allowing herself to be taken for punishment, giving the others an opportunity to pass through. They discover Tian has apparently been working for Sammael, but then learn he was only pretending to do so in the hope of stopping him.

Diyu was created to work like a dynamo, with the suffering of the tormented dead creating an enormous amount of power. Now, however, there are no dead to torment and Sammael discovers his plan to gather power here is not working. Tian rescues Isabelle, and they join forces with the others to rescue Simon. Magnus discovers a way to save Ragnor from the chains binding him to Sammael. With Ragnor no longer in his power and Shinyun absorbing so much power that she becomes a demon in her own right, Sammael abandons Diyu and the Shadowhunters, with the help of Magnus and Ragnor, return to their own reality.

When they reach the New York Institute, Magnus is reluctant to hold Max at first, fearing that the evil he had briefly become in Diyu would spill over into this life. He is pushed to realize this self-doubt is unfounded. Alec's mother and her servant seem slightly worse for the wear after their time alone with Max, but both deny he had anything to do with it and insist they look forward to the next time. Jace agrees to take on a larger role with the Shadowhunter Institute, with Clary at his side. Magnus hands over the Book of the White to another warlock with instructions on where to hide it. Sammael, meanwhile, gathers all the Princes of Hell for a meeting. He points out that the Shadowhunters ran him out of Diyu because working together made them powerful. He suggests the Princes of Hell consider doing the same.