In Rome, American diplomat Robert Thorn (Gregory Peck) is in a hospital where his wife Katherine (Lee Remick) gives birth to a boy, whoâhe is toldâdies moments after being born. Robert is convinced by the hospital chaplain, Father Spiletto (Martin Benson), to secretly adopt an orphan whose mother died at the same time. Robert agrees, but does not reveal to his wife that the child is not theirs. They name the child Damien (Harvey Spencer Stephens). Soon after, Robert is appointed U.S. Ambassador to the United Kingdom. Mysterious events plague the Thorns: large black dogs congregate near the Thorn home; Damien's nanny publicly hangs herself at his fifth birthday party; a new nanny, Mrs. Baylock (Billie Whitelaw), arrives unannounced to replace her; the five-year old Damien violently resists entering a church; and zoo animals are terrified of Damien.Father Brennan (Patrick Troughton), a Catholic priest, tries repeatedly to warn the Ambassador about Damien's mysterious origins, hinting that Damien may not be human. The priest later tells Robert that Katherine is pregnant and that Damien will prevent her from having the child. Afterward, Brennan is impaled and killed by a lightning rod thrown from the roof of a church during a sudden storm. Upon returning home, Katherine tells Robert that she is pregnant and wants an abortion. Learning of Father Brennan's death, photographer Keith Jennings (David Warner) begins investigating Damien. He notices shadows in photographs of the nanny and of Father Brennan that seem to presage their bizarre deaths. Photos of Keith also show these shadows. Keith shows Robert the photos and tells him he also believes that Damien is a threat and that he wants to help Robert. While Robert is away, Damien knocks Katherine over an upstairs railing to the floor below, causing her to miscarry.Keith and Robert travel to Rome to investigate Damien's birth. A fire destroyed the hospital records and the maternity and nursery wards five years earlier; most of the staff on duty died in the fire. Robert and Keith trace Father Spiletto to St. Benedict's Abbey in Subiaco, where he is recuperating from his injuries. Stricken mute, blind in his right eye and paralyzed in his right arm, Spiletto writes the name of an ancient Etruscan cemetery in Cerveteri, where Damien's biological mother is buried. Robert and Keith find a jackal carcass in the grave, and in the child's grave next to it, a child's skeleton with a shattered skull. These are Damien's unnatural "mother" and the remains of the Thorns' own child, murdered at birth so that Damien could take his place. Keith reiterates Father Brennan's belief that Damien is the Antichrist, whose coming is being supported by a conspiracy of Satanists. A pack of wild dogs, similar to ones seen near the Thorn's mansion, drive Robert and Keith out of the cemetery.Back in London, Mrs. Baylock persuades a nurse to allow her access to Katherine, who is heavily sedated and under police protection. Once inside the room, Mrs. Baylock pushes Katherine out of the window, causing her to fall into an ambulance truck, killing her. Robert and Keith travel to Israel to find Carl Bugenhagen (Leo McKern), an archaeologist and expert on the Antichrist. Bugenhagen explains that if Damien is the Antichrist he will possess a birthmark in the shape of three sixes, under his hair if nowhere else. Robert learns that the only way to kill the Antichrist is with seven mystical daggers from Megiddo. Appalled by the idea of murdering a child, Robert discards the daggers. When Keith tries to retrieve them, he is decapitated by a sheet of window glass sliding off a truck, matching the shadow across his neck which had presaged his death.Returning home, Robert examines Damien for the birthmark, finding it on the child's scalp. Mrs. Baylock attacks him and, in the ensuing struggle, Robert kills her. He loads Damien and the daggers into a car and drives to the nearest church. Due to his erratic driving, he is followed by the police, who arrive as he is dragging the screaming child to the altar. An officer orders him to raise his hands and stand away. Robert raises the first dagger and the officer fires his gun. The double funeral of Katherine and Robert is attended by the President of the United States, who now has custody of a smiling Damien. Just before the credits roll, Revelation 13:18 Appears "Here is wisdom, let him that hath understanding, count the number of the beast, for it is the number of a man and his number is 666."
Output: Question: Who does Robert kill?, Answer: Mrs. Baylock