An outlaw gang steals an army wagon full of repeating rifles and hightails it for Mexico. Mackenzie and his men cross the Rio Grande and steal the wagon back. Trying to hide the wagon's contents from the Mexican federales, Mackenzie drives the wagon towards the U.S. alone, but is captured by a Mexican colonel. When the outlaws kidnap the Mexican colonel's daughter and offer to trade her life for the rifles, Mackenzie offers to help the colonel rescue the young woman and wipe out the gang in the process.