Warning: This synopsis contains plot spoilers
It's the Swinging Sixties, and even the BBC is relaxing its standards. But as the permissive revolution sweeps Britain, one woman is horrified, and about to make history.
In the prosperous town of Claverley, 53-year-old housewife and art teacher Mary Whitehouse lives in a well-ordered world where people are polite and go to church on Sundays. But even here, she can see cracks appearing in the Britain she loves. Mary's determination to do something about it is spurred by a program about pre-marital sex at tea-time, and the discovery of some of her pupils mimicking sexual activity after watching television.
Mary's campaign starts with television, and more specifically the BBC and its Director-General — Hugh Carleton Greene, the man she believes is allowing the disintegration of the nation. Supported by her husband Ernest, her great friend Norah, and other supporters, she begins the Clean Up TV Campaign, to rid the BBC of the "tide of filth."
She begins quietly with a letter of protest, and when that is ignored, she organizes a petition and a public meeting. Mary's activism has little effect on Hugh Carleton Greene, who is equally determined to modernize British television and keep his program content uncensored. At first, he regards her with derision, and Mary is portrayed unflatteringly in a new comedy series, Swizzlewick.
Mary is sent a script for an episode of Swizzlewick that contains scenes she considers a breach of the obscenity laws. Through her efforts, the scenes are censored — her first triumph against the BBC. She follows this by delivering a petition to the House of Commons, and then announces the formation of the National Viewers' and Listeners' Association (VALA),
Her next victory is when Lord Hill becomes Chairman of the Board of Governors of the BBC. Lord Hill receives a letter from Mrs. Whitehouse objecting to the lyrics of the Beatles' song I Am the Walrus scheduled to be broadcast in family viewing time. When Lord Hill asks Greene to cut it, Greene refuses. The song is transmitted, uncensored. Mary continues bombarding Lord Hill on a variety of matters. In doing so, will she push Greene over the edge?