As the newest students at the conservative St. Francis Academy, Mary Clancy (Hayley Mills) and Rachael Devery (June Harding) find themselves in constant conflict with Mother Superior (Rosalind Russell). Though she doles out punishment for their numerous pranks, the Mother Superior tries with a grim yet sympathetic determination to turn these high spirited youngsters into presentable young ladies -- without embarrassing faithful members of the flock like Mrs. Phipps (Gypsy Rose Lee). Heeding the sisterly advice of Sister Celestine (Binnie Barnes) and Sister Constance (Camilla Sparv), Mary and Rachel learn to modify their nun-edeviling escapades. But just when there appears to be no hope for the school's two most unruly girls, they begin to change. Remaining at St. Francis throughout a Christmas holiday, Mary is touched by the sisters' celebration while Rachel, after initially doubting her sincerity, follows her example. So, when one of the girls ends up bringing her joy to the world as the other brings hers to the Church, Mother Superior feels a sense of triumph at their graduation.