What is the main conflict in The Talbot Odyssey by Nelson DeMille?

A group of lawyers and retired spies, including George Van Dorn, Patrick O'Brien, Tom Grenville, and Peter Thorpe, hears rumors of a Russian plot to take down the United States. Katherine Kimberly is another one of these lawyers, the daughter of former spy Henry Kimberly who was killed in action during World War Two. An English Colonel comes to her one day and gives her a letter that claims that a diary has been found containing new information on Talbot. Talbot is a double agent placed high within the intelligence community of the United States who has betrayed his country to the Russians since World War Two and has never been caught. The Colonel never arrives at the designated meeting location, and the diary is never found.