After putting Fox News through the ringer with Outfoxed: Rupert Murdoch’s War on Journalism, muckraking video maker Robert Greenwald turns his attention to the retail giant Wal-Mart, exposing its pitiful treatment of workers, ruthless conquest of small businesses, and canny exploitation of governments both local (subsidies for new store construction) and federal (Medicaid for its working-poor employees). Like the Fox documentary, this makes withering use of the corporation’s own imagery, contrasting sunny Wal-Mart commercials with the testimony of outraged competitors and former employees; it’s crude but effective, and completely defensible. 95 min.