Based on a true-life tale, Breach follows veteran FBI agent Robert Hanssen (Oscar winner Cooper). After 25 years of service, his actions have raised suspicions among his superiors. Eric O'Neill (Phillippe) is an ambitious young agent who’s assigned to pose as Hanssen's new clerk and observe his business dealings. Although O’Neill sees this job as a low-level task, he soon realizes the extent of his boss’s crimes and discovers that he has been selling secrets to the Russians. The rookie agent soon finds himself caught in one of the biggest security breaches in US history, forcing him to resort to ingenious tactics to reveal the truth and bring down the suspect.