The recent low-budget hit Paranormal Activity has been laughably hailed by delusional critics as “the most frightening movie ever made”, but it barely scrapes the foothills of the hair-raising ghastliness depicted in The Exorcist. William Friedkin’s demonic-possession shocker was released in 1973, but even today you wouldn’t want to watch it without keeping a large brandy and the off switch within easy reach. This documentary in 5's series The True Story, which tries to find rational explanations for movies, sought to explore the real-life story that the movie, and prior to that William Peter Blatty's novel, was based on.