Anna Wintour is high fashion's coolest and perhaps most powerful player. Her dynamic editorship of American Vogue has seen her mythologised for both her commercial vision and her chilly, inscrutable demeanour. RJ Cutler's irreverent and beautifully observed doc catches all of Wintour's buttoned-up mystique, and gets to the heart of what ruffles it – chiefly Vogue creative director Grace Coddington, a flamboyant foil to the boss' froideur.