Inspector French is the central figure of almost 30 many books by Mr. Crofts. This book is the beginning of the series, where a reader meets the detective genius for the first time. Inspector French is commissioned to help solve the case of diamond robbery and the murder of a diamond merchant in Hatton Garden. There are many people to suspect, and before Inspector French solves the case, he has to unravel many mysteries and follow up many false clues.