Surveillance expert Harry Caul (Gene Hackman) is hired by a mysterious client's brusque aide (Harrison Ford) to tail a young couple, Mark (Frederic Forrest) and Ann (Cindy Williams). Tracking the pair through San Francisco's Union Square, Caul and his associate Stan (John Cazale) manage to record a cryptic conversation between them. Tormented by memories of a previous case that ended badly, Caul becomes obsessed with the resulting tape, trying to determine if the couple are in danger.