New York has become too small for two circuses. The "Western Circus" and its owner Mulligan, competitor of "Barnum", decide to make a tour in the American West. But in the country of rodeo, the task is difficult, especially as Zilch, says D.D (diamond tooth), a notable from the corner, does not see the arrival of the circus with a good eye. Lucky Luke will do everything to make rodeo and circus live together. He will arrive for his ends with the birth of the "Zilch & Mulligan western show"!