At one point in “Io Capitano,” a deeply moving drama about an odyssey through unknown lands, the Italian director Matteo Garrone undermines his own realism, to startlingly lyrical effect. Seydou (Seydou Sarr), a sixteen-year-old from Senegal, is one of several African migrants who have been walking for hours across a great stretch of the Sahara Desert, bound, they hope, for Italy. Some distance behind him, an older woman (Beatrice Gnonko) collapses from exhaustion, wailing, “Aidez-moi! Aidez-moi!” (“Help me! Help me!”) Seydou runs back to help, offering the woman water from his canteen and urging her to keep walking. But his cousin, Moussa (Moustapha Fall), tells him to keep moving, and suggests there’s nothing more that Seydou can do. If they lose sight of their party up ahead, they’ll...