One tragic night, Caine survives a brutal assault when a demonic entity known as a Key saves his life. With his chance at a normal life long gone, Caine must accept his fate as a Key Holder possessed with remarkable supernatural power. But this power comes with a price. The Key inhabiting Caine has its own goal: the eradication of every other Key. Caine must harness the strength of his Key to find his place in the hidden world of hostile Exorcists, sinister Keys, and a network that spreads far beyond his hometown of Derry, Northern Ireland.