Luke Stone is an agent with the FBI.
He is not a typical FBI Special Agent. More accurately, he is a key member of an "elite, secretive department" within the law enforcement organization, tasked with handling situations far outside the norm and to do so both discretely and covertly.
We meet Stone really at two times of his life. Initially there are a series of ongoing adventures which take place while he has been in that elite department mentioned called the Special Response Team (SRT) for quite some time. Then there are a few prequel adventures describing the "Forging of Luke Stone" which depicts the adventures a ten-year-younger Stone goes through as he makes the transition of an already veteran Delta operative into the newly formed SRT.
In the prequel stories, Stone is 29 years old, fairly recently married to Rebecca and, as that time period begins, soon to be a father to a baby boy they will name Gunner. The year is 2005 and Delta operatives like Stone are very busy in both Iraq and Afghanistan but the need for a civilian Delta-style rapid response team to take on at home the sort of missions that Delta was handling around the globe had become apparent to the President. He selected Delta Colonel Don Morris to start such a group, attached to the FBI. Morris would have his pick of team members and top on that list is his protege, Luke Stone.
In the stories set in the current times, Stone is ten years older and, at least at the beginning, still with Rebecca and still very much the proud father of Gunner. 'Becca's willingness to continue to put up with Stone's being called away at any time has weighed too heavily for too long and it is clear something has to change. Unfortunately for the family, as much as Stone wants to be there for his family, either his sense of duty or his love of the excitement (depending on who you ask) will keep that from happening.
But stepping into the picture is the first female Vice President who will help Stone start a whole new chapter as she gets an unwelcome promotion.