The Cheyenne are convinced that newly-arrived settlers have been killing the buffalo they traditionally depend on. When the Native Americans surround the 20th Cavalry's fort, Lucky Luke realises that someone who bears a grudge against Colonel McStraggle has conveniently provided them with firearms. Worse, the traitor also knows the layout of the fort. Lucky Luke does his best to try and break the deadlock before the soldiers starve. As in other Lucky Luke volumes, readers discover different aspects of the United States' conquest of the Wild West.