John Wells is an agent for the CIA.
Born and raised in Montana, son of a skilled surgeon who had expected his son to follow his example, Wells opted for more adventure and joined the military. From there he was recruited by the CIA. After training, and probably some field experience, he was chosen for an incredibly difficult and self-sacrificing assignment - declaring his hatred of his own country and joining the most rabid of terrorist organizations, Al Qaeda.
As the series begins, he has been working as an undercover agent for more than five years, joining months before the 9/11 attacks. When the Twin Towers fell, Wells, who had been nowhere near enough to the top to have an inkling of the event, was hit with incredibly strong emotions. On the surface he had to celebrate the attack on the Great Satan while inwardly he loathed all those around him for the murders and blamed himself for not having figured it out. Wanting very much to get out but determined to stay to help fight from within, Wells had to walk a terribly thin line. Too much zeal would be suspicious while not enough would get him killed.
The series' best aspect is the intriguing attitude that comes over the leaders who send a qualified man into the field, let or make him work undercover for months or years, and then wonder whether the man can still be trusted. Has the agent become a follower of the cause, passing on harmful disinformation? And even if they do finally listen, what do you do with the agent afterwards? Try to "reindoctrinate" him? Send him back? Discard him?
That is what Wells finds out the hard way and the series deals largely with how he deals with that. Even after he returns, though, the missions change but the dangers do not and Wells finds himself constantly walking into danger to keep others safe.
Note: the first book in the series, The Faithful Spy, was the prestigious Edgar Award winner in 1997.