When Christopher Brooke is arrested under Regulation 18B in June of 1940 a slow process of personal disintegration begins. His subsequent years of detention as a supporter of Oswald Mosley's British Union of Fascists leads from imprisonment in Liverpool's notorious Walton Gaol, to 'special interrogation' at Latchmere House - the MI5 headquarters where Col. 'Tin-Eye' Stephens dispenses his own peculiar brand of psychological torture - to Peveril Camp on the Isle of Man. Mary Byrne, an Irish farm girl from County Cavan, arrives in England and is hired as housekeeper/nanny in the Brooke household where she bonds with Cynthia Brooke and her three children. Mad Hatter charts the slow unravelling of a marriage and the tightening of its children in the devastation of post-war England as the story of the Brooke family moves inexorably to a tragic conclusion. The denouement shows Christopher's descent into religious mania and his increasing isolation until his suicide in 1950, leaving Cynthia Brooke with her unresolved grief.