Mr Templeton

In the short story The Terrible Catastrophe, (later chapter 15 of The Big Four), Mr Templeton was an elderly gentleman who lived with his family at his house, Elmstead, in Herfordshire. His nurse, Nurse Mabel Palmer consulted Poirot because she was worried that he was being poisoned by someone. She suspected his much younger wife Mrs Templeton and the attending physician Dr Treves. When Poirot found antimony in a sample of Templeton's soup provided by Nurse Palmer, he decided to hurry to Hertfordshire. Later, Poirot would discover that he was being led into a trap laid by the Big Four. Templeton's son was Number 4 (Claud Darrell) himself and, most probably, the entire Templeton household were Big Four operatives.