This is the third novel in the Gray Man series and events from previous books are alluded to, but all that you really need to know is that Court Gentry is a former assassin for hire who is now hiding out in South America after being wanted dead by everyone from Russian criminals to Interpol. He becomes entangled with a Mexican family who are being targeted by a Mexican drug lord and the book is about his efforts to protect them.
After an explosive start the pace slows down, but it picks up again and the final third of the book is particularly enjoyable. It's very much a "suspend your disbelief" kind of read: the villain is laughably unbelievable, the romantic sub-plot has zero plausibility and the ability of the small family unit to withstand attack is highly unlikely. Despite these facts, it's entirely readable because the action is swift, the hero is likeable and the action scenes are written in such a way that you can tell Greaney has done his research.
It's a constant issue with thrillers these days, trying to strike the right balance between fast paced action that will keep us relentlessly turning the pages, whilst still giving us characters we can care about and a plotline that doesn't become entirely ludicrous. Greaney probably errs too much towards the former camp for my liking, but if you want a fast paced, disengage-your-brain read, you could do a lot worse.