In Rome, American diplomat Robert Thorn is told that he and his wife Katherine's newborn infant has died. Moments later, the hospital chaplain, Father Spiletto, urges Robert to secretly adopt an infant whose mother has died in childbirth. Robert agrees, but does not inform Katherine that the child is not their own. They name him Damien.

The family moves to London when Robert is appointed U.S. Ambassador to the United Kingdom.

During his fifth birthday party, Damien's nanny Holly climbs the roof of the Thorn estate and ties a noose around her neck. Before hanging herself in front of everyone, she tells Damien it's all for him.

A large Rottweiler appears near the Thorn home. Holly stares into the dog's eyes just before she kills herself.

The day after Holly's death, Mrs. Baylock arrives unannounced and presents herself as Damien's new nanny.

As the church looms into view, Damien is thrown into a rage, pulling his mother's hair and clawing at her face. The Thorns are forced to drive hurriedly away.

When Mrs. Baylock becomes Damien's new nanny, she finds the Rottweiler on the grounds of the Thorn estate and brings it into the house as a watchdog for Damien.

Damien's presence terrifies the animals, and the scene climaxes with a group of baboons attacking the car.

After warning Robert that Damien is the son of Satan, Brennan is impaled by a church spire while trying to take shelter in the church from a freak rainstorm.

Kathy becomes more and more disturbed by Damien's behavior. When she discovers that she's pregnant, she asks Robert to agree to an abortion because she doesn't want another child like Damien.