Michael Pretorius is an agent for the DIA.
At least he was for twelve years prior to the first book in the series. Each of those years had been as an operative in the field, usually behind the Iron Curtain, hunting down facts, creating a network of natives unhappy with the regime or just in need of extra money. Years of putting his life at risk knowing that the men in Washington were growing complacent with his risks. When an East German woman who had worked for him asked to be brought out, he tried but she stepped on a mine and died and he spent weeks in a Soviet hospital being interrogated. When he was finally let go by the Soviets, he was offered a desk job or retirement. He chose the latter.
As the series opens, Pretorius is living a comfortable life in Devon where he is thought of by the locals as the eccentric American. It is a peaceful life, free from the stress of his previous career. He was content for the first time in years and did not miss the excitement or the danger at all. He was not looking for anything but the rest of his life being left alone.
Then he gets a visit from the Head of Station for the CIA. They need a trained, experienced operative who was not and never had been on their books. The ex-DIA was the perfect candidate. He knew he should say no but he did not.
Working for Pretorius is the fact that as a blown agent, especially one who had been held by the Soviets for so long, most people were buying his retirement. No one considered that he might go back into the field, especially for a rival agency. Pretorius was probably as surprised as everyone else.