Rex Bishop, a perpetually down-on-his-luck P.I., has spent his life avoiding his father's shadow, mostly by underachieving. Now that he's learned his father is dead and he's been tapped by Dante Grimm to help prevent an apocalypse of unimaginable proportions, he puts his meager skill set as a former mediocre boxer and low-rent investigator to the test in trying to find a missing artifact, a piece of metal that is purported to have been one of the nails from the Crucifixion. With the aid of a plucky young woman assigned to him as a Squire, Rex follows the trails where they lead, forced to battle creatures and powers he never dreamed existed, never quite certain what is real and what isn't.
But Rex soon discovers that reality is a matter of perspective, and things are often not what they seem. He learns the artifact he's tasked with finding is the not the true object of his quest at all, but merely a key that unlocks something that cannot easily be put back. With people supposedly on his side not being completely honest, and others who've tried to kill him now the ones warning him of dangers, Rex must navigate uncharted waters filled with mythical beasts and mystical forces, ultimately finding himself forced to engage in a contest older than recorded history, a competition of martial strategy where life and death are the lowest of the stakes, a game that Rex learns he must find a way to win, at any cost, or take all of humanity down with him in defeat.