The film centers around Harry Caul (Gene Hackman), a highly skilled but emotionally isolated surveillance expert in San Francisco. He's hired to record a seemingly innocent conversation between a young couple in a public square. But as he obsessively listens to and replays the audio, he begins to suspect that the people he recorded may be in danger — or that his work may lead to their deaths.
Caul becomes consumed by guilt and paranoia as he realizes he might be complicit in something sinister. The line between reality and delusion starts to blur as the film progresses.