Few who saw it will ever forget the horror classic, The Exorcist. What most people don't realise is that the movie was based on a true story.

In incredible interviews, priests who performed a 1949 exorcism on a 14-year-old Maryland boy tell the galvanizing story of their battle to free his soul.

We trace the history of exorcism from the Middle Ages, when it was commonly practiced, to current day. Ancient texts and accounts reveal the secrets of the Catholic ritual of exorcism, while religious scholars explore the evidence of exorcisms in other cultures.

See how many examples of possession are cured by modern psychiatric counselling, and meet debunkers who claim that mental illness is at the root of all such cases.

From Medieval Europe to the disturbing claims of some fundamentalist groups that use exorcism as a justification for brutal beatings of women and children, this is a captivating look at exorcism through the ages.