The Bunker, Volume 4 — Joshua Hale Fialkov

As Grady Potts' political star rises, Daniel finally realizes what he must do to ensure he doesn't poison the world's population, and it isn't pretty. Natasha is on hand―actually, both Natashas are―but the consequences of his actions may change the future for the worse. Heidi is still imprisoned, but just like Billy, she's got an advocate on the outside. In this explosive final volume of the critically-acclaimed series, the fate of the world will be decided.