Quick little synopsis.
I forget the first names so I will refer only to them by their last.
Mr. Thorn and his wife are in Italy.....she is pregnant. She gives birth to a baby but it dies. Mr. Thorn replaces his dead son with another child and never tells Mrs. Thorn. This child is Damien, the antichrist. Damien is surrounded by many people who protect and serve him as they are Satan worshipers. Every main character in the movie dies except Damien. His father learns who he is and attempts to kill him but is shot before he can. Damien survives.
That was really broad and brief but that is the basic jist of it.