It is the crisis that has rocked continents and shaped global politics. The mass migration of people has dominated headlines and had far-reaching consequences on the Brexit vote and the election of Donald Trump. As record numbers of people have been displaced by conflict, harrowing stories have emerged of families divided and abuse at the hands of smugglers. The artist Ai Weiwei, whose own father was exiled from his homeland, has now directed a film tracking the scale of the migrant crisis across 23 countries, one of the biggest challenges facing humanity today.