John Davidson is a former agent for the CIA.
Nicknamed "The Baptist", Davidson was already nearing retirement age when, in a purge of the CIA, he was forced out, rumored to be a Soviet mole. The irony in that claim was the fact that Davidson was really ousted because he had made too many enemies fighting to uncover the real traitors in the Agency. After several decades of solid service rising to the rank of Chief of Counterintelligence, he was pushed out.
With the money he had saved and the pension he was given on his dismissal, Davidson returned to a small dairy farm outside Leesburg, Virginia. With his dog, now nearing the end of his life, and 300 cows, he was almost content to let the world go its own way.
When trouble calls in the first novel, Davidson, a young 68 years old, realizes that he really isn't finished. He may not have the moves of the young guns but he has more brains than most and more experience than just about all.
That is not to say that he doesn't still have some moves, though, as the experience agent who first came asking for Davidson's help found out quickly enough, when the cold barrel of a gun from a Davidson he never knew was there was followed by an even colder "what do you want?"
As the series progresses, Davidson's retirement becomes a thing of the past as he proves his usefulness to the President many times over, winning many accolades and the Medal of Freedom. This makes him the most trusted man the President knows but also keeps him from enjoying his dairy farm as much as he would like.