Mary was once the wife of Jeff Deacon and is now the wife of Will Thoday. She was once accused of playing a role in the theft of the emeralds, but was found innocent. However, her brush with the law has tarnished her local reputation. She is a law-abiding woman with a strong conscience - she insists that Will remarry her when she discovers that her marriage to him was invalid, as Deacon had been alive and still her legal husband when she married Will. Discovering that she had, technically speaking, committed bigamy and adultery horrifies her, and she is very grateful when Lord Peter addresses her as "Mrs. Thoday" even when it is known that, at that point, she is not legally married to Will.