Former German soldier Gabriel Keller is serving a life sentence for murder. Told by a judge that he is too dangerous to ever be released, Keller is a man without purpose. Surviving, not living.
Department 89 receives a credible threat that the French President will be assassinated on his upcoming state visit to Germany. Major Sophie Decker is ordered to ensure nothing happens to the President while on German soil. The leader of the assassination plot is inside the same prison as Keller. But they don't know who he is.
So Decker offers Keller a deal. Help track down the plotters, help to foil the assassination attempt, and gain his freedom as a reward.