Harry Caul (Gene Hackman), an expert in secret taping of conversations receives an assignment by the Director (Robert Duvall) to spy on his wife, Ann (Cindy Williams), and her lover, Marc (Frederic Forrest). Harry is reluctant to turn the tapes over to the Director when he discovers on them pieces of the conversation that indicate that the Director may have them killed if he learns from the tapes that his wife is cheating on him. Harry has a history of having others murdered by his clients when they received information from his surveillance. Tortured by the past and uncertain about what to do with the results of his present assignment, Harry keeps the tapes but avoids the Director. His paranoia deeply affects his ability to trust others and causes him to live almost entirely isolated from others, unwilling to allow them into his personal life. But by not destroying the tapes, he cannot escape those who want that information and must confront the paranoia that threatens to destroy his life as well as the consequences of what others do with secret information he has retrieved.