Mrs Halliday

In the short story The Lady on the Stairs (later, chapters 5 and 6 of The Big Four), Mrs Halliday was the wife of English scientist John Halliday. The police were inclined to believe that her husband's disappearance in Paris was voluntary, and probably because of some woman he was having an affair with. Mrs Halliday, however, was convinced that it was foul play as she knew that her husband was only interested in his scientific work. She was grateful to Poirot for believing her.