ather Brown is visiting St Agnes convent where he is chaplain when novice Sister Mary Magdalene is poisoned,leading to the father speaking to the convent's vintner Sister Boniface,a fan of detective stories who is keen to help him solve the murder. Then newly-arrived Sister Paul,an austere,dominating nun,is also poisoned and it would seem that she was a secret drinker who ate pear drops to mask the smell of alcohol and was killed by poisoned wine. She had also been the strict warden at a home for unmarried mothers some years earlier. Father Brown and Sister Boniface independently find a link between one of the mothers and a current inhabitant of the convent,but is this the killer? And do the convent's philandering young gardener and his cowed wife know more than they claim?

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