For the second time in her career Dame Beatrice Lestrange Bradley is called upon to solve the mystery of murder in a convent. After a series of anonymous letters have been received by the Sisters and also by members of the staff at a school run by the nuns, the supposed writer of the letters is found drowned in the school pond, together with her own Family Bible.

At the inquest a verdict of accidental death is returned, but Dame Beatrice regards this as unsatisfactory for reasons which she passes on to the local Inspector of Police. With some help from him and some evidence supplied by two of the schoolchildren, she unmasks a heartless and cowardly murderer.