Caroline Carmichael is a terrorism expert for the CIA.
For several years prior to the start of this series, Carmichael was an analyst for the CIA working in the Office of Russian and European Analysis, a division with the unpleasant acronym of DI/OREA. While she was knowledgeable about a variety of items in that region, her specialty had been the German government and she was an expert on the current Chancellor. This made her also aware of the radical group called 30 April, led by a Croatian named Krucevic already wanted for acts of ethnic cleansing throughout Europe. The group, dedicated to eliminating all non-Aryan people from Europe, was responsible for violent attacks all over that continent and the carnage are getting bigger and fiercer.
She was happily married to a CIA operative named Eric, a field agent who had met her while she was taking a paramilitary course for the Agency. It was there he gave her the nickname of Mad Dog. They had been married for several years before they were temporarily separated with her returning to Langley on assignment and he heading to Istanbul. Then a bomb went off onboard his jetliner, killing him and shattering her happiness.
The series starts with explosion two years in the past but never far from her mind. She had been easily persuaded to move to the Agency's Counterterrism Center where Carmichael made herself a leading expert in terrorism in general and this group in particular all for the day she might be able to avenge her dead husband as well as stop such acts from happening again.