Magnus Bane, a centuries-old High Warlock, has taken possession of one of the great relics of the supernatural world, a powerful spell book of dark magic known as The Book of the White. There are many who want to claim it for themselves.

After the Mortal War, where the part-human and part-angel Shadowhunters teamed with the part-human, part-demon Downworlders to fight against an army of demons, Magnus and his new lover, the mortal Alec Lightwood, celebrate their survival and victory by escaping New York City's supernatural battlefield and touring the world. However, the world won’t leave them alone.