When 12-year-old Regan displays unusual and disturbing behaviour, and after all medical options have been exhausted, her mother becomes convinced the girl is possessed by an evil spirit. An exorcism is sanctioned, and ageing priest Father Merrin is dispatched to carry it out. But at what cost for all? William Friedkin’s horror classic shocked audiences on first release. 50 years on, it remains just as compelling, memorable and disturbing.