Without giving away any more than the back cover blurb, the premise is a nuclear terrorist attack on Washington, and the details of the detection of such a threat and the response to it are so precise that a U.S. government inquiry was launched into how Flynn got his information (answer—he has lots of fans in the alphabet soup agencies within a megaton or so of the Reflecting Pool). While the earlier novels in the Mitch Rapp chronicle are best read in order, you can pick this one up and enjoy it stand-alone: sure, you'll miss some of the nuances of the backgrounds and interactions among the characters, but the focus here is on crisis, mystery, taking expedient action to prevent a catastrophic outcome, and the tension between those committed to defending their nation and those committed to protecting the liberties which make that nation worthy of being defended.
As with most novels in which nuclear terrorism figures, I have some quibbles with the details, but I'm not going to natter upon them within a spoiler warning block because they made absolutely no difference to my enjoyment of this yarn. This is a thriller by a master of the genre at the height of his powers, which has not been dated in any way by the passing of years since its publication. Enjoy!