When his wife gives birth to a baby who dies shortly after, the father takes a priest's advice and adopts an orphan baby without telling his wife that it isn't hers. Five years later, little Damien is a happy little well-adjusted kid. So what if his nanny hangs herself at his birthday party? So what if animals are terrified of him? So what if a priest insists that little Damien is the antichrist?
Good movie? No.
This thing is almost triumphantly silly. The dialog, the story, and especially the music conspire to make it nearly impossible to keep a straight face for more than a few minutes at a time.
The decapitation is great, though. It's a real "let's see that again" kind of movie moment. I must have watched that shot four times before I finally decided to let the rest of the movie play.