Perpetually seventeen, Callum the witch has been in the service of Lucifer, collecting souls, as his own soul is indebted to the devil until he completes 666 tasks for him. Summoned to Hell on assignment, Lucifer tells Callum that completing his next task will wipe the remaining tasks from his ledger and he will return Callum’s soul to him upon completion. All Callum must do is ensure his latest charge, Augustus, makes it from London to America, by the next blood moon. As they journey across the sea, Callum grows to care for Augustus "Auggie". Auggie seems unlike any mark Callum has ever been charged with and Callum wonders why such a noble boy would sell his soul to the devil. Upon reaching New York, Callum discovers that Auggie never sold his soul to the devil at all but is an innocent. But is Callum willing to forfeit his own freedom to save Auggie?