From Wikipedia: "Crossley (Alan Bates), a mysterious travelling man invades the lives of a young couple, Rachel and Anthony Fielding (Susannah York and John Hurt). Anthony is a composer, who experiments with sound effects and various electronic sources in his secluded Devon studio. The couple provides hospitality to Crossley but his intentions are gradually revealed as more sinister. He claims he has learned from an Aboriginal shaman how to produce a "terror shout" that can kill anyone who hears it unprotected."