Robert Thorn, a wealthy American, sits in the lobby of a Catholic hospital in Rome, Italy, on 6 Jun at 6:00 a.m. after his newborn son dies at birth. Fearing that his wife, Katherine Thorn, cannot handle loss of the infant, Robert allows Father Spiletto to secretly replace the deceased baby with another newborn whose mother died in childbirth. Years later, Robert is appointed ambassador to the U.K. and the family moves into an English country manor. At a large birthday party for the Thorn’s five-year-old son Damien, a news photographer, Keith Jennings, snaps a photograph of Damien’s nanny, Holly. Minutes later, Holly is confronted by a large black Rottweiler. Later, Damien hears Holly yelling his name. The party guests look to the roof of the house where they see the nanny with a noose around her neck. Declaring she is killing herself for Damien, Holly jumps off the roof. As Katherine and Robert console Damien, the boy waves at the black dog. A few days later, Robert walks into the U.S. Embassy followed by reporters and he knocks Jennings’ camera to the ground. Robert apologizes and offers to pay for the broken camera, but Jennings declines. Father Brennan unexpectedly arrives at Robert’s office, announcing that Robert must accept Jesus as his savior or the ambassador is doomed. Robert calls security, but before they arrive, Brennan explains that he was present at Damien’s birth and the boy’s real mother was a jackal. As Marines escort Brennan out of the embassy, Jennings snaps the priest's photograph. At home, Robert and Katherine are perplexed when Mrs. Baylock, a new nanny, arrives without solicitation, but Baylock explains an agency sent her. However, Baylock later tells Damien that she was sent to protect him. Despite Baylock’s protests, the Thorns take Damien to a wedding. When the boy sees the church, he attacks Katherine. Later, Robert is shocked to see the black dog at Damien’s nursery door. Mrs. Baylock declares that she thought the stray would make a good watchdog, but Robert insists she get rid of the animal. The next day, Katherine takes Damien on a drive through Animal Park, but all the animals run away from the boy except the baboons, which attack the car. That night, Katherine confesses to Robert that she is having irrational thoughts about Damien and begs her husband to find a psychologist. Days later, at a rugby match, Brennan tells Robert that Katherine is in great danger and the two men agree to meet the following day at Bishop’s Park. As Brennan leaves, Jennings photographs from afar. At Bishop’s Park, Brennan insists Damien is the Antichrist and must be killed. He warns that Katherine is pregnant and Damien will kill the mother and child. When Damien is Robert’s sole heir, the baby will kill Robert as well. Brennan begs Robert to visit a small town called Megiddo to see an old man named Carl Bugenhagen, but Robert says he never wants to see Brennan again, and leaves. Just then, a storm begins, pelting Brennan with leaves and hail. Lightning strikes as Brennan races to a nearby church for shelter, but all the doors are barred. A bolt strikes the church’s lightning rod and it falls, impaling Brennan to the ground. When Robert returns home, Katherine sobs she is pregnant and wants an abortion. At that moment, Jennings phones to tell Robert to look at the evening newspaper, which features a picture of the impaled Brennan. Later, Robert talks to Katherine’s psychologist, who tells him that Katherine is fantasizing that Damien is evil and not her child. The doctor recommends the abortion, but Robert refuses to consider it. Meanwhile, on the second level landing at the Thorne house, Katherine climbs onto a small table to prune a plant while Damien rides his tricycle into the table, causing his mother to fall and lose her unborn baby. At the hospital, Katherine begs Robert not to let Damien kill her. Robert returns home to find the black dog guarding Damien’s nursery. When Jennings phones, begging Robert to come to his flat, Robert sees photographs of Brennan and Holly, Damien’s deceased nanny. In the image of Holly, there is a flaw that looks like a noose and the picture of Brennan also shows a spear-like flaw impaling his body. Brennan’s autopsy photograph reveals a birthmark in the shape of “666,” the sign of the devil from the Biblical Book of Revelations. The men investigate Brennan’s apartment, which is plastered with Bible pages and crucifixes. Brennan’s diary contains a five-year-old newspaper article about a comet changing shape on 6 June at 6 a.m., the exact time of Damien’s birth. Jennings reveals a photograph he took of himself in a mirror that has a similar flaw running through his neck. Back in Rome, Robert and Jennings discover that the hospital where Damien was born burned in a fire five years earlier. All the records were destroyed, but a nun tells Robert that Father Spiletto, the priest who gave him Damien, now lives in a monastery nearby. The men find Spiletto, but the priest was badly maimed by the fire, and can no longer speak. When Robert demands information about Damien’s biological mother, Spiletto writes the name of an ancient Etruscan cemetery. That night, Robert and Jennings break into the cemetery and find Damien’s mother’s grave next to the grave of a child. Exhuming the woman’s grave, they find the bones of a jackal. The child’s grave contains the skeleton of Robert’s natural son with a smashed skull. As Robert realizes his son was murdered at birth, a pack of black Rottweilers attack. Escaping over a spiked fence, Robert slips and impales his arm, but Jennings pulls him free. Robert phones Katherine and reports that his assistant will take her to Rome. When Katherine hangs up, Baylock throws her out the window onto an ambulance ten floors below. After Robert learns of Katherine’s death, Jennings informs him that the ancient city of Megiddo is in Israel and that “Megiddo” is another word for “Armageddon.” Robert remembers the name of Carl Bugenhagen, the man Brennan told him to meet. In Israel, Bugenhagen sends Jennings away and gives Robert a set of knives with crucifix handles, explaining that Damien must be killed on consecrated ground. Robert asks if there is any way to prove the boy is the Antichrist and Bugenhagen describes a birthmark in the shape of “666.” When Robert insists Damien does not have the mark, Bugenhagen tells him to check under the boy’s hair. Disbelieving Bugenhagen, Robert declares to Jennings that he will not kill Damien, and throws the knives away, but Jennings runs to retrieve them. However, a nearby truck loses its emergency brake, rolls down the hill, hits a rock, sending glass flying through the air and decapitating Jennings. Robert flies back to England with the knives, sneaks back into his home and traps the black Rottweiler in the basement. While Damien sleeps, Robert cuts the boy’s hair and sees the mark of the beast. As Robert recoils, Baylock leaps on him and tries to claw his eyes out, but he knocks her unconscious and grabs Damien. As they head downstairs, Damien grabs a wall sconce and the electrical shock sends them tumbling. Robert regains consciousness, and carries Damien into the kitchen, where Baylock attacks him with a carving fork. He stabs her in the neck, then speeds out of the driveway with Damien. Robert's car almost hits a police officer, who radios for backup and gives chase. Robert takes Damien into a church and places him on the altar. As he prepares to stab Damien, the boy begs for his life, and Robert hesitates. The police arrive and shoot Robert before he can stab Damien. Robert and Katherine are given a formal ambassador’s funeral. After the ceremony, Damien is led away by the U.S. President and First Lady.