As soon as they stepped off the boat in New York, Irish immigrants Jim Leary and his brother Billy were conscripted into the Union Aero Corps, forced to fight in a civil war they had no interest in. Ten years later, with the war finally over and America divided into two nations, each brother believes the other died in combat. Jim Leary, now calling himself "Captain Gallagher", has taken to the air over the southwestern American territories in a stolen airship, leading the life of a pirate, raiding trains, stage coaches, and even other airships, the bane of both the Union and the fledgling Confederate States of America.

A consortium of businessmen, tired of losing money and goods to Gallagher and his pirate cohorts, has hired a former Confederate air admiral to hunt them down. Now Gallagher finds himself pursued by an overwhelming force led by the infamous Garrett "Butcher" Prescott, the man he blames for his brother's death, and the beautiful and accomplished Justine Arthurs, an airship pilot, former Pinkerton agent and Union spy. Gallagher thinks he's got a plan that will bring down the Valkyrie, Prescott's massive dreadnaught. What he doesn't know is that the chief engineer on the Valkyrie is his long-lost brother.