As the Cold War enters its final act, the CIA is sent scrambling when a rendezvous with a prized asset goes horribly awry. CIA officer Sandy Grimes begins her investigation into the source of the problem and quickly discovers that this lost asset is just the tip of the iceberg. Little does she know that this is just the beginning of a decade-long mole hunt, and the mole is one of her cohorts - Aldrich Ames.

The CIA scores a major break when Vitaly Yurchenko, a high-ranking KGB officer, defects to the US. Assigned as the lead in the debriefing, Aldrich Ames engages in a verbal cat-and-mouse game to determine whether he is in danger of being ratted out. Ames considers taking drastic measures when Yurchenko reveals that he knows of a mole in the CIA. Meanwhile, Sandy frets over the safety of a beloved and esteemed asset, General Dmitri Polyakov.

After Yurchenko's shocking disclosure, the CIA and FBI mobilize to bring the brilliant Edward Lee Howard to justice. Despite Howard being fingered as the mole, Sandy remains concerned about potential agency leaks and works tirelessly to prove that her asset, General Polyakov, is attempting to establish contact with the CIA. With the rest of the CIA distracted by bigger matters, Ames develops an unlikely bond with Yurchenko during a trip to Canada.

After learning that a breach remains at the CIA, Sandy butts heads with Jeanne Vertefeuille, the senior officer conducting a thorough review of their operations. Jeanne's presence also rattles Ames, who feels increasing pressure from the Soviets to provide more information. On the home front, Ames feels even more pressure, caught between his love for his wife Rosario, and placating her spendthrift lifestyle.

As Sandy conducts her review of the agency's spycraft, she and Jeanne continue to clash. When a potential asset contacts the CIA claiming to have vital information about a communications breach, Sandy has her doubts about the man known as GTPROLOGUE. Could this be part of a larger campaign by the Soviets to obfuscate the truth? Meanwhile, Ames learns that his request for a transfer to Rome has been approved.

Having put aside their differences, Sandy and Jeanne move full steam ahead with their mole hunt. The investigation experiences a serious setback, though, when Sandy learns that General Polyakov has been compromised. Through sound recordings, Sandy experiences the brutal interrogation of Polyakov who remains heroic to the end. As Sandy deals with her devastating loss, Ames prepares himself for a high-stakes polygraph test.

After the fall of the Berlin Wall and collapse of communism, the CIA task force still remains on the hunt for the cause of the assets' compromise. Sandy becomes convinced that Ames is the most likely suspect but has a difficult time convincing the rest of the task force. After living glamorously in Rome, Ames finds it increasingly difficult to maintain that lifestyle and keep his identity hidden.

In the series finale, despite the task force being fully convinced of Ames' guilt, Sandy seeks more definitive proof to convince the Justice Department to proceed with their own investigation. As the hunt closes in on Ames, Sandy's commitment to the job pushes her husband's patience and commitment to the brink. After Ames is finally caught and brought to justice, he mulls over a major decision - to go down defiantly or to confess and protect his wife and child.