Muhammad Ali is a natural performer. More to the point, starring in his own autobiopic, The Greatest, he brings to it an authority and a presence that lift John Marshall’s production above some of the limitations inherent in any film bio.The film gets off to a fine start with newcomer Phillip MacAllister playing the young Cassius Clay Jr displaying the engaging affrontery of a young talent so sure of himself that discretion in self-description knows no bounds. Plot follows Ali from his early career through formal discipline, professional conflicts and the controversial refusal to be inducted in the US Army. En route is Ali’s deliberate public baiting of Sonny Liston. Intercut are actual sequences from Ali’s major fights.