Michael Bryce is an elite bodyguard.
Darius Kincaid is an elite assassin.
Together they will, unwillingly, work together to first put away a notorious despotic ruler of Belarus for major war crimes and then some time later stop a powerful terrorist leader from bringing down the European Union.
When we first meet Bryce, it is two years before the first main adventure and Bryce is, as he tells us several times, a "Triple-A rated executive protection agent" for many of the world's rich and famous and endangered. He has put together an impressive group of associates who work with stopwatch precision to get powerful but threatened people from Point A to B alive and unharmed. He has a catchphrase that encompasses his approach to his work: "Boring is better".
Unfortunately, when an assignment went absolutely smooth and perfect until the very end when a sniper's bullet dispatched Bryce's client, Bryce's career took a major downhill slide and the main events start up, he is working for the second-rate level of clientelle and hating every minute of it. We learn that this 40-something year-old operative is recently separated from his long-term Interpol girl friend and really disgusted with the world and himself.
Kincaid, on the other hand, is extremely pleased with himself and his position in life. Well, the very latest hitch is not so appealing considering this likely 60-something internationally wanted paid assassin is finally in FBI custody and is making a deal to get his much loved wife, Sonia, immunity. Kincaid, we learn right away, has been a very busy man all his life earning apparently a very good living killing. When Bryce mentions later on that Kincaid has dispatched around (I believe) 140+ people, Kincaid laughs that it is closer to 240.
These two men are not friends. They are not even friendly. They would be very happy trying to eliminate the other but life forces them to work together and when they do, a whole lot of mayhem and destruction ensues and an impressive number of very nasty people get eliminated.