Mr Petherick is a solicitor in St. Mary Mead, and member of the Tuesday Night Club.

In The Thirteen Problems, he is the fifth person to describe a mystery to the Tuesday Night Club. He presented the story Motive v. Opportunity about a tampred will.

In Miss Marple Tells a Story, Miss Marple says that at the time of the events she is narrating (some time ago), a certain Mr Petherick visited her with a client Mr Rhodes who had been accused of killing his wife. Miss Marple says this Mr Petherick as an old friend who attended to all her legal issues. This Mr Petherick could well be the same one as in "Thirteen Problems". In refering Mr Rhodes' problem to Miss Marple, Mr Petherick likened it to the value of having two points of view. He said, "In a case of illness one likes two points of view—that of the specialist and that of the family physician. It is the fashion to regard the former as of more value, but I am not sure that I agree. The specialist has experience only in his own subject—the family doctor has, perhaps, less knowledge—but a wider experience." They had already engaged a specialist Sir Malcom Olde, K.C. but wanted to consult Miss Marple as the equivalent of the family doctor with less specialised technical knowledge but wider experience.

In "Tells a Story" Miss Marple also says that after Mr Petherick's death, his son took over his business.

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