Mrs. McGinty's Dead (1951)

Poirot, disillusioned by the "senseless cruel brutality" of modern crime, pays no attention to the sad case of Mrs McGinty, an old woman apparently struck dead by her lodger for thirty pounds that she kept under a floorboard. When, however, he is asked by the investigating officer to take another look at the case to stop an innocent man going to the gallows, he realises that things may not be as simple as they first appear to be.