The Republic of Texas, 1862. Fourteen year old Gilly Ward — mechanician, horsewoman, and darned good shot with a rifle — wants nothing more than to ride the range and care for the steam lines which power the Republic. When global war threatens, however, Gilly's parents are summoned to England. Soldiers from New Spain soon arrive in west Texas, intent on kidnapping Gilly. Chased across the Republic, Gilly and her friends team up with a wandering knight and discover a devastating weapon the Spanish have built in the Indian Territory. Threatened by hostile soldiers, enemy warships, and altered beasts Gilly slowly uncovers secrets of her own. With each new challenge she reveals nearly superhuman capabilities which surprise her as much as her friends. Aided along the way by a traveling preacher, a reclusive scientist, an airship captain, and a beautiful spy, they race against the clock to find President Houston and thwart the invasion of Texas.