Sound surveillance expert Harry Caul is hired to track a young couple, taping their conversation as they walk through San Francisco's crowded Union Square. Knowing full well how technology can invade privacy, Harry obsessively keeps to himself, separating business from his personal life, even refusing to discuss what he does or where he lives with his girlfriend Amy. Harry's work starts to trouble him, however, as he comes to believe that the conversation he pieced together reveals a plot by the mysterious corporate "Director" who hired him to murder the couple. After he allows himself to be seduced by a call girl who then steals the tapes, Harry is all the more convinced that a killing will occur, and he can no longer separate his job from his conscience.