Older movie. Maybe black and white. American family moves to London. Father opens a shop. Father runs afoul of local gangster by writing down his license plate number after gangster threatened him. Father buys a gun for protection. Tries to scare off gangster with unloaded gun. Gangster learns that gun is not loaded and beats father to death. Mother and son move back to america. Someone asks son – did you learn from this experience? Son either says or thinks. yes, always make sure your gun is loaded.