Emily Snow is twelve years old, supporting herself and her younger brother on the streets of Victorian England by selling watercress. One early winter morning on her way to buy supplies, she encounters a piskie—a small but very sarcastic fey creature that has been cornered by a group of the Black Sidhe, piskies from an opposing clan. She rescues him and unknowingly becomes involved in a war between the Seelie and the Unseelie, two opposing factions of fairies that have been battling each other throughout the long centuries of human history, with London—and England itself—as the ultimate prize. When the Invisible Order—a centuries-old secret society of humans that has protected mankind from the fey's interference—gets involved, things really start to get complicated. Now she is the central figure in this ancient war that could permanently change Earth. With no one to trust, Emily must rely on her own instincts and guile to make the right choices that could save her family and all of mankind.

With humans threatened by otherworldly creatures, orphans Emily and William Snow, and their friends—the pickpocket Spring-Heeled Jack and the wisecracking Corrigan—find themselves two hundred years in the past, trapped in the London of 1666. Desperately in need of help, they go in search of Sir Christopher Wren, who was head of the Invisible Order, an organization dedicated to fighting this threat. But Wren’s never even heard of the Order and has no interest in their story. Stranded, the four cannot agree on their next step. But they’ll have to decide quickly, because their enemies are on the move and the Fire King is ready to attack and burn London to the ground. Set against the Great Fire of London, The Invisible Order, Book The Fire King picks up right where Rise of the Darklings left off, weaving adventure, history, and legend into a thrilling, heart-stopping story.