Nancy, Bess, and George can hardly believe their eyes when they arrive at the Black Hills Buffalo Ranch. Aside from the awesome natural scenery, Mount Rushmore National Memorial—with four larger-than-life US presidents—towers nearby. But the girls have no time for sightseeing: They've come to help Bess's friend Kincaid Turner find out who stole a baby buffalo named Justice from her father's ranch.

At first, stealing a buffalo seems like a strange thing to do, but as Nancy and her friends investigate, they find a whole herd of suspects. Antoinette Francoeur, an eccentric animal rights activist, makes a habit of freeing caged creatures. Badger Brady, a rival buffalo rancher, has been making threats—with an ax. And some archaeologists in the area aren't happy about the girls' snooping. Now it's Nancy's turn to round up the clues and stop a buffalo thief in his tracks.