Hayden Stone is an agent with the CIA.
The fact that such a statement might be true would have made him laugh, if not growl, for many years. When we first meet him, the statement is making his CIA handler, Claudia, do the growling. The problem is that Stone has for many years been a Special Agent for the FBI until his recent retirement. To some people, like his new handler who has been ordered to make use of him, that is working with the enemy. Forget the 'other side'. The FBI and the CIA are not friends.
Nevertheless, Stone is now at loose ends. He was retired but not that old in his opinion. His marriage has ended because his wife decided he no longer fit into her career plans. His two grown children were on the West Coast in college and on their own. He needed a new direction.
The CIA, at the same time, had just had an agent killed in the south of France. Since it seemed obvious that he was eliminated because someone knew he was an agent, what was needed was someone working for them who could investigate the murder, like a detective such as Stone, and someone not part of the Agency officially who would be an unknown, again such as Stone. Stone also seemed like a good idea because he was not just a cop, as important as that was, but also someone who had extensive knowledge in counter-terrorism, having just come back from a grueling deployment in Afghanistan (this adventure takes place shortly after the Towers fell).
It would be the start of a new and exciting career for Stone, if he can survive.