Col. Bullock, commanding officer of Fort Lawrence, orders the immediate execution of two “savages” from the Sioux tribe for brawling with soldiers.
Captain Young opposes these orders, stating that his enquiries have shown that it was his own men who provoked the fight.
Col. Bullock will have none of it, but agrees to postpone the execution while Cap. Young and his men collect the supplies being delivered by the Cannonball Express.
Also on board the Cannonball Express are Casey’s wife and son who are visiting Capt. Young’s wife Ruth, and Mr Grant from the Bureau of Indian Affairs who is investigating complaints about Col. Bullock’s handling of the Sioux.
While Casey, his family and crew are at Fort Worth, the Sioux hold the fort under siege, and it is down to Casey to save the day.