Harry Ryder is an agent with the CIA.
At the start of the series, Ryder was retired from the spy business and had been for five years. It had taken its toll and cost him his marriage, or as good as. "Give up the job or give up me" had been his wife's ultimatum as she moved her and her teenage son out of their Georgetown home and moved to California. After a year or so of independence, he decided she had been right. He quit and moved to join her but now there was a cold friendship. Affection still and a need for each other, especially after the son went off to college and medical school. Affection and need but no longer any heat or passion.
Back when he was a spy, he had been very, very good at it. In a career that spanned from the latter years of WWII through much of the Cold War, Ryder was a master of disguise who could take on the roles of numerous people as he went about his assignments. He was a genius at blending in and becoming a person one never really noticed.
He had thought that those days were over and that he did not miss them and that he was happy being with this wife at long last but when he is approached again for work and his wife tells him first that if he goes he should not come back and secondly she was hoping he would choose to leave, he chose to leave. Having so chosen, he quickly sees how much he missed the action.
He also sees that while he is still good at his job, he is now in his fifties and not as young and spry as he once was, which means to stay alive he has to be even better than he once was.