Nick Barber is an agent for the CIA. Sally Stein is a Special Agent for the FBI.
The two-book series follow two cases they share, meeting in the first and continuing and intensifying their relationship in the second.
Barber has worked for the Agency for a very long time, At or around fifty, he still possesses impressive physical abilities but a slight paunch is present and he doesn't move like he once did. The gray hair on the side of his head and the lines on his face show he has paid his dues. These dues were extracted in numerous hotspots around the world and he is unofficially credited with several tactical eliminations in the performance of his duties. He no longer feels he has to prove anything to anyone, least of all himself.
Sally Stein also is a veteran of many years but with the FBI. Divorced mother of two teenage boys who live with their father, she admits the constant moving and long-term assignments cost her the marriage but she has also garnered an impressive record. In her late thirties, she is not feeling aches and pains yet but knows they are coming. She still retains her good looks and her figure but doesn't care if they are appreciated like she might once have.
They are thrown together in the first assignment where they instantly feel animosity for each other. Barber doesn't like working with others and Stein doesn't like working with the Agency. As they share adversity and save each other's life, that attitude changes. He still is a loner and she still dislikes the CIA but they grow to like and trust and value each other more than they expected.