In 1965, the world's attention was drawn to a small city in the state of Alabama, where Dr. Martin Luther King and a growing swathe of Civil Rights protesters were marching for African-Americans' right to vote. Ava DuVernay's excellent film - more urgent than a straightforward biopic and particularly relevant in light of racial tension in America today - offers insight into how the selflessness of a group of individuals changed the course of US history.