Christie revealed in her 1977 autobiography that the basic idea of the novel was given to her by her brother-in-law, James Watts of Abney Hall, who suggested a novel in which the criminal would be a Dr Watson character, which Christie considered to be a "remarkably original thought".[23]: 342
In March 1924 Christie also received an unsolicited letter from Lord Mountbatten. He had been impressed with her previous works and wrote, courtesy of The Sketch magazine (publishers of many of her short stories at that time) with an idea and notes for a story whose basic premise mirrored the Watts suggestion.[15]: 500 Christie acknowledged the letter and after some thought, began to write the book but to a plot line of her invention. She also acknowledged taking inspiration from the infamous case of the unsolved death of Charles Bravo, who she thought had been murdered by Dr James Manby Gully.[24]
In December 1969 Mountbatten wrote to Christie again after having seen a performance of The Mousetrap. He mentioned his letter of the 1920s, and Christie replied, acknowledging the part he played in the conception of the book.[25]: 120–121
