Satan's son has arrived on Earth and He's not about to let human parents get in the way. When his wife Katherine's pregnancy ends in a stillbirth in a Rome hospital, U.S. diplomat Robert Thorn substitutes another baby, whose mother died. Little Damien thrives, but, at his fifth birthday party, his nanny mysteriously dies, Father Brennan also expires after warning Thorn that he has adopted Lucifer's son. While sinister new nanny Mrs. Baylock assiduously protects Damien, Thorn's fears escalate when photographer Jennings shows him pictures from Damien's party with marks suggesting how the nanny and Brennan would die. Thorn seeks out Bugenhagen, an exorcist who confirms Damien's identity and tells Thorn that the only solution is to kill his adopted son. As the bodies pile up, Thorn tries to do his duty, but trust the law to get in the way of saving the world from future Armageddon.