A young FBI agent named Eric O'Neill is assigned to work under senior agent Robert Hanssen, who is suspected of being a mole for the Soviet Union. O'Neill is told his task is to gather evidence to expose Hanssen's espionage activities. Initially, O'Neill is skeptical of the allegations against Hanssen but gradually grows suspicious as he observes Hanssen's peculiar behavior and strict adherence to conservative Catholic values. O'Neill's investigation is complicated by Hanssen's erratic demeanor and their developing personal relationship, as Hanssen begins to mentor O'Neill, adding layers of complexity to the undercover assignment.

As the pressure mounts, O'Neill must navigate the dangerous web of lies and deceit while carefully managing his interactions with Hanssen to prevent arousing suspicion. Throughout the process, he deals with the emotional strain of the double life he is leading, which impacts his marriage and personal life significantly. With growing tension, O'Neill becomes increasingly aware of the stakes involved, realizing that successfully unmasking Hanssen is critical for national security. Ultimately, the story illustrates the inner workings of the FBI and the formidable challenges posed by counterintelligence operations, all while examining themes of loyalty, betrayal, and the moral ambiguities faced by those tasked with preserving national safety.