Dr Quentin

In the short story The Yellow Jasmine Mystery (later, chapters 9 and 10 of The Big Four), Dr Quentin is the locum for Dr Bolitho, the local doctor serving the community at Market Handford in Worcestershire. Quentin attended Mr Paynter who said he felt unwell after a dinner. Later Paynter was found dead with his face burnt in the fireplace.

Poirot showed that in fact Paynter had been murdered by Dr Quentin. He had pretended to inject Paynter with strychnine to treat his heart condition but in actual fact had injected him with an alkaloid “yellow jasmine”. The Quentin who did the killing was really Claud Darrell, "Number 4" of the "Big Four" consiracy in disguise. He had killed Paynter in order to get hold of a manuscript of a book written by Paynter called "The Hidden Hand of China" which might have compromised key details of the conspiracy.

Poirot surmised that there probably was a real Dr Quentin somewhere, perhaps on holiday. The arrangements with Dr Bolitho for locum were done through correspondence.