Ai Weiwei's 'Human Flow' captures the plight of refugees on an epic scale

The images, whether captured by drone or phone, come patiently and powerfully in Ai Weiwei’s large-scale documentary “Human Flow,” an artful, solemn globe-hop epic of the refugee crisis with an unfortunate cast of millions.

The famed dissident Chinese artist, an assortment of film crews in tow,...