Mary Clancy (Hayley Mills), with her best friend Rachel Devery (June Harding), succeeds in driving the Mother Superior (Rosalind Russell) to distraction with an endless number of practical jokes. Just when there appears to be no hope for the school's two most incorrigible students, they get a little help from Sisters Celestine (Binnie Barnes) and Constance (Camilla Sparv). During Christmas break, Mary remains at the school and is touched by the Sisters' celebration of the Yule. Rachel first doubts Mary's sincerity, but later follows Mary's example. Their graduation is both a relief and celebration for the Mother Superior.