"A man known simply as Calculus has approached the FBI claiming he has a list naming several Americans who are supplying confidential government information to the Russian secret service. All he asks in exchange for the list is a quarter of a million dollars for each traitor the FBI nabs. But then Calculus informs the FBI that he’s been recalled to Moscow, and the Bureau suspects the worst: the Russians are onto him, probably have access to his list, and will be after the traitors unless the FBI can find them first. The FBI knows they have to stay one step ahead of the Russians, and, without knowing exactly who is on the list, keep the operation quiet. Once again, Vail is their man. He’s the perfect guy for this kind of under-the-radar investigation. But finding Calculus and his list of turncoats isn’t going to be easy. In fact, it’s going to be downright deadly."