Buck Horne and his faithful horse Injun were once the heroes of many a Western movie in the early days of Hollywood, but when tastes changed, Buck found his talents no longer required. Down on his luck, he went to work in a rodeo exhibition that was appearing in a New York coliseum, giving exhibitions of roping, fancy shooting, and the riding tricks that made him famous. With twenty thousand people in the stands, a group of celebrities including detective Ellery Queen in the boxes, and a full cohort of newsreel movie photographers recording the event for posterity, Buck and forty-one cowboys and cowgirls gallop around the track, whooping and firing their six-guns — until the former movie star is shot in the heart and trampled under the galloping hooves.
Suspicion falls on many of the rodeo's performers and staff, and even on some of the celebrities, but one crucial and baffling point must be explained before anyone can be arrested. Even though all 20,000-odd people and the entire arena are searched, and the entire event can be reviewed on film, the specific murder gun cannot be found. Ellery Queen works his way through the details of the murderer's clever plot to set a trap and reveal two astounding surprises — the identity of the murderer and the hiding place of the gun.