The King of Plagues brings back some baddies from a prior book as the Department of Military Sciences discovers the existence of the Seven Kings. They are a powerful cabal bent on causing chaos throughout the world and profiting from it. The Seven Kings use their knowledge of the upcoming chaos they will cause to make money on various fronts as wars and catastrophes impact the world markets. Ruled over by "The Goddess" there are seven members of the cabal each with an advisor who acts as their "conscience." It turns out that they have a vacancy and are in need of a new King of Plagues.
The King of Plagues - Joe Ledger #3 builds some story arc