As a young girl Colleen McCabe asks a priest in confessional "What is sin?" Thirty years later she is found out for practising it. An ex-nun,she leaves the convent because she becomes disillusioned with spiritual matters and goes into teaching, being appointed headmistress of the John Rigby School in London. Along with a small coterie of chosen staff members to act as her spies,she misappropriates half a million pounds from school funds which she spends on luxury goods and a trip on the Orient Express. Meanwhile the school suffers,having to use ancient text books and pupils as cleaners. She is tried,although admitted to hospital for depression on the trial day, and sentenced to five years in jail, later reduced to four. The film alternates dramatized scenes of Colleen's misbehaviour with interviews with those who knew her.