In February, 2001, Robert Hanssen, a senior agent with 25 years in the FBI, is arrested for spying. Jump back two months: Eric O'Neill, a computer specialist who wants to be made an agent is assigned to clerk for Hanssen and to write down everything Hanssen does. O'Neill's told it's an investigation of Hanssen's sexual habits. Within weeks, the crusty Hanssen, a devout Catholic, has warmed to O'Neill, who grows to respect Hanssen. O'Neill's wife resents Hanssen's intrusiveness; the personal and professional stakes get higher. How they catch Hanssen and why he spies become the film's story. Can O'Neill help catch red-handed "the worst spy in history" and hold onto his personal life?