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 (Lee Remick) pregnancy ends in a stillbirth in a Rome hospital, U.S. diplomat Robert Thorn (Gregory Peck) substitutes another baby, whose mother died. Little Damien (Harvey Stephens) thrives, but, at his fifth birthday party, his nanny mysteriously dies; Father Brennan (Patrick G. Troughton) also expires after warning Thorn that he has adopted Lucifer's son. While sinister new nanny Mrs. Baylock (Billie Whitelaw) assiduously protects Damien, Thorn's fears escalate when photographer Jennings (David Warner) shows him pictures from Damien's party with marks suggesting how the nanny and Brennan would die. Thorn seeks out Bugenhagen (Leo McKern), 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.
