"Filth" is the surprising story of Mary Whitehouse and her David-and-Goliath type struggle with the BBC in the 60s. With Julie Walters starring as Whitehouse and Hugh Bonneville playing her arch-enemy, BBC Director-General, Hugh Greene, their clash of values forms a portrait of a time that shaped the TV we watch today. Forty years ago, her battle with the BBC started over The Beatles' use of the word "knickers."