Lucky Luke and Jolly Jumper arrive in Grass Town and make the acquaintance of Smith I, Emperor of the United States! A local rancher gone a bit loony after getting rich, he's given his cowboys fancy uniforms and replaced their Colts with sabres. The citizens of Grass Town think he's great fun, write him letters in the names of European monarchs and publish his harmless proclamations in the newspaper. But is he truly harmless? And what will Luke do if something upsets the relationship between the Emperor and his not-so-loyal subjects?