Expert surveillance operative Harry Caul is renowned for his exceptional skill at audio spying and his commitment to privacy. His latest assignment seems straightforward enough: to record a conversation between a young couple as they meander through San Francisco's bustling Union Square. Armed with state-of-the-art technology and accompanied by his assistant Stan, Harry manages to capture a cryptic dialogue that hints at a potential tragedy. Obsessive about his craft and consumed by a past experience where his eavesdropping may have cost lives, Harry becomes fixated on deciphering the couple's ambiguous words, trying to determine their underlying meaning and intent.

As Harry delves deeper into the recording, layering sound upon sound to isolate phrases, he is plunged into a vortex of paranoia and moral ambiguity. Struggling with a guilty conscience and the fear of a repeat of his previous mistake, Harry begins to suspect that the couple may be in danger. His secretive nature, combined with the weight of potentially life-threatening information, strains his relationships with those around him, fueling his isolation. As he confronts his employers for more information, Harry finds that the stakes are higher than he anticipated, compelling him to make choices that risk his safety, his professional ethics, and his personal sense of right and wrong.