Warned by a soothsayer at a traveling fair that something evil is about to come to Maryneal, sixteen-year-old Delah Nix chalks it up to carnie theatrics and ignores the woman's suggestion to leave before dark. But then her sister is savagely attacked on their way home, and her murder leaves the small farming community stunned. The authorities scramble to place the blame on a deranged man, the carnival workers, and even the nearby Indigenous community.

Delah, however, starts to suspect that something supernatural is afoot when a childhood friend, bitten in an attempt to save Delah's sister, gains powers and a thirst for the county's livestock. As much as he is enjoying his super speed and perfect hearing, Delah senses there will come a point of no return and begins to desperately search for a way to save her friend before he hurts himself or the girl they both love. With the full moon approaching and no salvation in sight, Delah is faced with an unconscionable decision: If she can't find a cure, she'll have to kill the boy next door.