Christabel Carstairs has inherited the estate of her late, well-to-do father with the exception of his celebrated collection of Ancient Roman coins, which go to her elder brother. As an art student, she becomes infatuated with a male model and impulsively steals a coin from the collection as she believes its likeness of Augustus Caesar Augustus resembles the young man in question. When a blackmailer tries to coerce money from her to keep the theft secret, she tuns to Father Brown and Flambeau for help.