"Darwin's Nightmare," Hubert Sauper's harrowing, indispensable documentary is framed by the arrival and departure of an enormous Soviet-made cargo plane at an airstrip outside Mwanza, Tanzania. Though Mr. Sauper's investigation of the economy and ecology around the lake ranges far and wide - he talks to preachers and prostitutes, to street children and former soldiers - he keeps coming back to a simple question. What do the planes bring to Africa? A.O. Scott reviews the film in the New York Times (free subscription required to view).