Magnus Bane and Alec Lightwood are settling into domestic life with their son Max when the warlocks Ragnor Fell and Shinyun Jung break into their loft and steal a powerful spell book. Realizing that Ragnor and Shinyun are being controlled by a more sinister force, Magnus and Alec set out to stop them and recover the book before they can cause any more harm. With the help of Clary Fairchild, Jace Herondale, Isabelle Lightwood, and Simon Lovelace (who is fresh from the Shadowhunter Academy), they track the warlocks to Shanghai.

But nothing is as it seems. Ragnor and Shinyun are working at the behest of a Greater Demon. Their goal is to open a Portal from the demon realms to Earth, flooding the city of Shanghai with dangerous demons. When a violent encounter causes Magnus’s magic to grow increasingly unstable, Alec and Magnus rally their friends to strike at the heart of the demon’s power. But what they find there is far stranger and more nefarious than they ever could have expected…