Colton Barnes, Royce O'Connell, and Charlie Porter return to face a new threat to the United States as the possibility of a second civil war hangs in the balance.

In an alternate history of the United States where the Civil War lasted 10 years, a brilliant scientist is kidnapped by a crazed madman known as John Conway, the United States Marshals are called upon to prevent the impending disaster. To track down Conway, Marshal Colton Barnes enlists the help of Sheriff Royce O'Connell, who has been living in seclusion in an attempt to forget his painful past. Together, they set out with a team of Marshals to hunt down Conway. But nothing can prepare O'Connell or the Marshals for what awaits them.