As Procter and Sam investigate, they arrive at a shortlist of suspects made up of both Procter's closest friends and fiercest enemies. The hunt soon requires Procter to dredge up her own checkered past in service of CIA, placing her and Sam into the sights of a savvy Russian spymaster who will protect Moscow's mole in Langley at all costs, even if it means wreaking bloody havoc across the United States.