Technically the plot is about the redemption of the doubting Father Karras, who sacrifices himself to save a life, but it's the sense of evil in the film that stays with you. The "good" characters seem bewildered and powerless. Regan's mother refuses to even tell the girl's father what's been happening.
The demonic Regan punches her mother and other investigators and (offscreen) brutally murders a man. She also mutilates herself, vomits blood, and in a famous moment, rotates her own head completely around. Another character falls fatally out a window and down a flight of steps.
Nothing seductive or titillating, but still intense, as the possessed girl lewdly propositions men, talks about oral sex, and is briefly shown rubbing a crucifix in her groin.
A little girl taken over by the devil doesn't hold back much in the cursing department.
Soft-drink signs, movie-company logos.
Drinking, Drugs & Smoking
Social imbibing, and a character is drunk at a party. Mention of pills and pot.