*"Oh, mother! That wretched woman! After all, she's dead and buried. Why don't you leave well alone?" enquired her son.
​"So said the ghost of Joan of Arc to George Bernard Shaw," Mrs. Bradley replied, with a chuckle.*

Mrs Bradley is called in when two boys escape from a model juvenile reformatory. It's not the first such incident - several years before, two inmates had also run away, never to be seen again. The reformatory cook-housekeeper is implicated, but an investigation turns up nothing in the way of solid evidence. Years later, the unhappy woman commits suicide.

Then her diary falls into Mrs Bradley's hands, and the mistakes and inconsistencies it contains prompt the canny psychologist/detective to follow the trail of years-old crimes.