Abby is a Ranger, part of an elite group who defend the border against Reapers - humans infected with a parasite that turns them into mindless cannibals. Rangers are immune to Reaper infection, and as one of the only female Rangers, Abby is expected to settle down and breed more Rangers - a fate she's keen to avoid. When she's ambushed on the plains, she's ready to go out with guns blazing - until a mysterious, handsome cowboy rides to her rescue.
Jake has his own motives for helping Abby, beyond aiding a damsel in distress. He's a Reaper, and while he's learned to wrest control of his mind from the parasite, the effects won't last without a permanent cure. And he needs Abby to get it.
Abby and Jake are natural enemies and unlikely partners. But when their search reveals a conspiracy between Reapers and the rich industrialists who own the mountain cities, they must work together to find the cure - or lose the border, and each other, forever.