Ghoulish deaths of three Catholic priests, the insides of their bodies seemingly burned away, baffles the London Metropolitan Police Department in 1920. Inspector John Nelson is requested by Commissioner Edward Allenby to handle the case. Soul-weary after his military service in World War One, John is hesitant to accept, thinking he is well past the prime of his career, but the commissioner believes that he is the best man for the job, and insists that he take the assignment.
John learns that Doctor Alex Milton, an enthusiastic occult scholar, has been chosen to help him on the case. After they view one of the grisly corpses in the morgue, Alex tells John about his research on the Gnostics and their power to weld mysterious fire, suggesting that this is the cause of the uncanny deaths. He also describes a secret ancient cabal known as the Brotherhood of the Snake, which is believed to have once possessed the Amulet of Solomon, a mystical relic that is a source of this power and can be traced to Biblical times. Alex claims that shadowy demonic beings, called Archons, are connected with it.
At first put off by Alex, John believes there is a perfectly rational explanation for the deaths. However, he soon comes upon haunting clues which lead him deeper into the world of the occult. After Sarah Wilford, a long-time friend of Alex, teams up with them, they come to discover that the Archons are trying to find the Amulet of Solomon and free their lord, Asmodeus, who is imprisoned inside of it. John comes to find that there is truth to what Alex has been saying, that evil forces are killing people with a supernatural energy.
The investigation leads them across the English countryside to find the Archons before they can retrieve the Amulet. They come upon an old, secluded monastery in Wales where they encounter monks who are with the Brotherhood of the Snake and who can summon the same otherworldly powers to fight the rancorous demons. Only with their help, can Asmodeus and the Archons be defeated, and the Amulet reclaimed... before the world falls into terrible darkness.
