The prussic acid poisoner always made it look like suicide. The bodies were always found in locked rooms. And each one had been married to lovely Lesley Grant, who looked 18, admitted to 28 but in fact may be a 41 year old serial poisoner, with a trail of corpses behind her. That's what Dick Markham found out the day before he got engaged to her. Sir Harvey Gilman, Home Office Yard pathologist, seeks Markham's help in proving - or disproving - that Leslie is the killer. But then Gilman is found dead - poisoned by prussic acid - behind the locked door of his cottage and Dr. Gideon Fell is called in to untangle the clues.