Six heavyweight champions carry this ESPN profile from the bare-knuckle era to the modern title picture: John L. Sullivan, the last of the bare-knuckle kings and boxing's first national celebrity; Jack Dempsey, whose brawling style filled arenas in the 1920s; Jack Johnson, the first Black heavyweight champion, whose title reign provoked riots and a manhunt across the color line; Joe Louis, champion through the Depression and World War II; Rocky Marciano, who retired undefeated; and Muhammad Ali, whose fights and politics made him the most recognized athlete of his century. The film moves chronologically through their careers, using each man's reign to mark a different era of the sport and the country around it, from segregated arenas to televised superfights. It stays close to the ring, tracking title fights and rivalries rather than biography for its own sake, and treats the heavyweight division as a running record of who America would and would not accept as its toughest man.