Lord Captain Stephano de Guichen, formerly of the Dragon Brigade, and his disparate group of friends who call themselves the Cadre of the Lost, are hired by the powerful Countess de Marjolaine, to find a Royal Armory journeyman who has mysteriously vanished, along with an invention that could revolutionize warfare. The Countess fears the invention may fall into the hands of their enemies. Always in need of money, Stephano and his friends undertake what they think is an easy job, only to discover they are being dogged by spies and targeted by assassins.
Meanwhile, Father Jacob Northrop, a priest of the feared Arcanum, and his Knight Protector, Sir Ander Martel, are dispatched to investigate the massacre of a hundred nuns at the Abbey of Saint Agnes. A lone survivor claims the nuns were attacked by demons from Hell.
Stephano and his friends take to the skies in their airship, the Cloud Hopper, still on the trail of the journeyman. Their route takes them near the Abbey of Saint Agnes. As they draw near, the Cloud Hopper comes under attack by what appear to be demons riding giant bats. Stephano teams with Father Jacob and Sir Ander and a dragon from his old brigade to fight the hellish forces.
After the battle, one question is on everyone's mind: Are these truly demons sent by the Evil One? Is this the Apocalypse?
As Father Jacob searches the Abbey to find the answer, he uncovers a startling secret that nearly gets him and Sir Ander killed. Stephano's search for the journeyman almost ends in disaster, as he and his friends encounter the deadly Freyan assassin, Sir Henry Wallace.
Schemes and tricks, lies and intrigues culminate in an exciting chase through the skies that comes to a shocking end, when friends and foes alike are caught up in the unexpected and terrifying conclusion.