In Rome, Italy, at the early hour of 6:00 AM on June 6, 1966, Robert Thorn (Gregory Peck), an American diplomat, rushes to a local hospital where his wife, Kathy Thorn (Lee Remick), is in labor. Upon arriving, he is devastated to learn that the infant she carried has been stillborn. In a twist of fate, a priest named Father Spiletto (Martin Benson) offers Thorn a chance to adopt a newborn boy, who is motherless. Torn between his grief and knowing the news would devastate his wife, he reluctantly agrees.
The child, named Damien (Harvey Stephens), is brought to England when Thorn is made the U.S. ambassador. During Damien's fifth birthday party, shocking events unfold when a nanny takes her life in a traumatic display, all while a menacing rottweiler observes. This unsettling incident is captured through the lens of a news photographer, Keith Jennings (David Warner), who begins to notice strange happenings around the Thorn family.
As new nanny Mrs. Baylock (Billie Whitelaw) assumes her role, it becomes clear she is concealing dark intentions. During a church visit, Damien exhibits fearful behavior that injures Kathy, sparking concern over his true nature. Thorn is confronted by Father Brennan (Patrick Troughton), who insists that Damien is the antichrist, born of a jackal, destined to wreak havoc on those close to him. His warnings are dismissed, yet an unexpected death of the priest highlights the danger surrounding them.
The tension escalates as various animals react violently towards Damien, and ominous signs lead Thorn and Jennings to investigate further into Damien's origins. They make harrowing discoveries in Italy, uncovering a dark plot involving their biological child and Damien's true lineage, complete with a grave representing a jackal. An air of desperation surrounds Thorn as he learns of Mrs. Baylock's sinister involvement.
As tragedy strikes, with Kathy's death orchestrated by Mrs. Baylock, Thorn wrestles with the truth of Damien's identity. In a race against time, he journeys to Megiddo, armed with daggers of destiny after seeking counsel from an archaeologist, Carl Bugenhagen (a character not in the original cast but essential to the plot). Yet, despite the profound implications, Thorn hesitates to carry out the unthinkable act against a child.
In a tragic turn of events, as Jennings meets a grisly fate, Thorn finds himself ever closer to confronting his son in a desperate bid to save the world from impending doom. The climax builds as he invades a local church, intentions clear but thwarted by law enforcement. The film culminates in a deeply haunting finale with Thorn and his wife's funeral at Arlington National Cemetery, where Damien is seen with a knowing smile, portraying a chilling fulfillment of evil prophecy.
