The contours of Muhammad Ali’s life are as firmly entrenched in the American memory as his fists were in the jaws of his foes: he won gold at the 1960 Olympics; he changed his name to join the nation of Islam; he won the world heavyweight title in a fight against Sonny Liston; he resisted the draft during the Vietnam War; he re-claimed the heavyweight title from George Foreman. Now, doing what he does best with a four-part documentary series, Ken Burns is filling in the unknown gaps of the legendary boxer’s well-known story and, crucially, reminding us that for the greatest of all time the challenges were not limited to inside the ring. —J.V.