As a man, Gabriel, and his dog are out hunting in southern Mississippi, the dog discovers and opens a small, unusual golden container while chasing a deer. When Gabriel locates the dog, it acts strangely before violently attacking him. After shooting the dog, Gabriel returns home, displaying the same unusual behavior and black eyes. After killing his family, he enters a neighboring house and begins killing the family there before being shot dead by the family's father, Marcus, who immediately gets the same black eyes and proceeds to murder the rest of his family.A demon hunter, Jebediah Woodley, hears news of the murders on the radio and travels to the small town to investigate, at the same time as FBI agent Evelyn Pierce, who grew up in the area and has been assigned to investigate possible domestic terrorism. The police chief, Dunham, and Pierce quickly dismiss Woodley's claims of demon activity and have him detained as Pierce leaves to interview a witness to the most recent series of murders. Woodley advises her to ask about the killer's eyes. The witness explains that Marcus entered her home and killed her daughter before being killed by her husband, who then suddenly killed their son; he attempted to murder her before she escaped. Pierce learns that that the husband's eyes turned black as he went into his murderous rampage and realizes Woodley's story is true.Woodley explains to Pierce and Dunham that this demon possesses the body of whoever killed its last host and is unusual in its high body count. The only way to defeat it is by killing it only after ensuring one's own death; his own father captured the demon when Woodley was a child by poisoning himself before killing the host, with Woodley holding onto the captured demon for decades before it escaped. As Woodley and Pierce investigate the last person to have killed the demon's host, Pierce is attacked by the demon. Instead of killing her, the demon shows an interest in her and attempts to get her to kill it, giving her horrible visions of an apocalyptic future. Woodley attacks it, but it escapes. Later, Pierce visits her old home and tells Woodley that as a child, she drowned in her pool, but was mysteriously brought back to life minutes later, and was later chased out of town by superstitious citizens.Dunham organizes a town hall meeting to warn citizens of the danger but is interrupted by the demon, who slaughters the congregation and rapidly changes bodies before Woodley can capture it. It flees to a nearby house, where it manipulates a young girl into killing it. Dunham flees from the town as Woodley enters a church, where the local pastor Erikson, who remains suspicious of him, accuses him and Pierce of being evil. Annoyed, Woodley attacks him and leaves, rejoining Pierce before encountering the father of the possessed girl, Emmett, who urges Woodley to help. Woodley and Pierce travel to Emmett's home, where he has locked his daughter in a backroom. Woodley convinces Emmett to poison himself and kill his daughter, ending the demon's trail as before. However, the FBI, who have been called there secretly by Pierce, storm the home before Emmett can carry out the killing, and Emmett dies uselessly; the head agent, Deacon Shepherd, has Woodley arrested for poisoning him. The possessed girl slaughters the FBI agents except for Shepherd, who mortally wounds a possessed agent before fleeing with Woodley and Pierce.The demon eventually possesses Shepherd, who once more goads Pierce into killing it. Woodley captures it and prepares to poison himself before killing it, like his father. However, he and Pierce are interrupted by Erikson, who has tracked him down with a mob of congregation members who believe Woodley is evil and has captured an innocent man. Erikson attacks Woodley as the mob frees Shepherd, who immediately begins to murder everyone present. Erikson kills him and is possessed. As the demon tries to kill Woodley, Pierce grabs a vest covered in grenades from a corpse and activates one before killing Erikson. Apocalyptic phenomena from Pierce's earlier visions begin to occur as a possessed Pierce levitates off the ground, apparently an extremely powerful host for the demon, but the grenade detonates and kills her. Woodley sadly captures the remains of the demon in a flask, which he drops into the ocean. However, the flask is quickly consumed by a great white shark, implying that the events will repeat in the near future.