Maria Black, headmistress-detective, is invited to the engagement celebrations of an ex-pupil - and becomes involved in the apparent suicide of the prospective bridegroom, Keith Robinson. The circumstances - death by hanging in a locked cellar - satisfy the coroner that Robinson took his own life. However, 'Black Maria' thinks otherwise, and sets out, with her companion 'Pulp' Martin, to prove how Keith really died. She uncovers a plan of cold-blooded murder. But who is the killer - and why was Robinson killed?