Surveillance expert Harry Caul (Gene Hackman) values his personal privacy and anonymity above all else. When he and partner Stan (John Cazale) are hired by a mysterious client known only as 'the director' (Robert Duvall) to follow a young couple, Harry deduces that the woman, Mary, is the director's wife, and the man an employee with whom she is conducting an affair. Harry becomes convinced that the director intends to murder the pair and, haunted by guilt from a previous assignment where the information he provided resulted in loss of human life, sets out to prevent the killing himself.