William Cowley is an agent with the American FBI. Dimitri Danilov is an officer with the Moscow Militia.
The time period for their first recorded adventure, the first time the two are brought together on a case, is 1992. The Soviet Union has fallen and the Russian Federation come into existence.
Danilov is a Colonel in the People's Militia for the Moscow region, an experienced and unusually dedicated policeman in a new order where the old set of rules and regulations was barely holding on as the environment, legal and political, changed rapidly. We learn right away that Danilov had been one of those eager for a change from the usual Soviet inefficiency, only to become saddened and disillusioned when the new was just like the old. Over time his position will change as he will make enemies and then friends in high places and will move to the new Organized Crime Bureau.
William Cowley is an experienced FBI special agent who has gradually moved up in promotion until he has reached his current position of director of the Russian internal desk, a place that he recognizes is certainly the best he is ever likely to achieve in the Bureau; it is also one he is pleased and surprised to have reached considering his trouble in London some time back [think his best friend and his wife, now both exes, and toss in a major drinking problem that he mostly refuses to admit].
The two men would never have met each other or learned to work so well together and even to become good friends had it not been for the murder of a young American woman in Moscow. Even that would not have been enough to draw them together except for the fact that the victim was the niece of a powerful U.S. senator who demands that the Moscow Militia make use of the offered help of the FBI. Both Cowley and Danilov will find considerable pressure from multiple sources but both will decide finding the truth and getting it known were the priorities.
After that, both Cowley and Danilov will find their lives changing, sometimes for the worse but largely for the better. They also find that their jobs, already quite difficult to do, is nearly impossible when dealing with politicians, mobsters, and spy organizations.