The screenplay sketches the turbulent political background of the story, so that we get a partial sense, at least, of the government’s neglect of its drought-wracked people. William’s story guides the film. As starvation looms, the boy pieces together his idea (inspired by his teacher’s pedal-powered bicycle headlight) for a wind-powered water pump. Ejiofor doesn’t race past the adversities on the way to the affirmative finale. For once, in a family-friendly, true-enough true story, the life-and-death conditions look and feel legitimate.