In 1824, Catherine, a Catholic spinster of 44, unexpectantly inherits millions. However, she doesn't use it to climb the social ladder or snare a husband; she uses it to fulfill a lifelong dream of building a refuge for the poor and sick of Dublin, Ireland. That an unmarried woman would dare propose such a thing is so scandalous, even her own brother calls it "Kitty's Folly." Society turns against her. The Church tries to take over. Catherine must defend her choices or lose not only her inheritance, but her reputation and life's calling.

One of the first women who seeks Catherine's help is Margaret, a maid in the house of Lord Montague, the loudest of Catherine's detractors. Daring to protect herself from his advances and rebel against his maxim of total obedience, Margaret is forced to flee for her life. She desperately approaches Catherine for help, setting off a series of events that haunt Catherine all her days and prompt a rule that holds today, in the real-life Sisters of Mercy.