Miss Amelia Butterworth (who was christened Araminta but feels that too weak and sentimental a name for a woman of her), a 50-something spinster, looks out her window at midnight and sees a man and a woman enter the darkened house of her vacationing next door neighbors. Though — she says — she is not an inquisitive woman, she continues to watch and sees the man leave alone. When the house is still dark the next day, she summons a policeman, and they find the body of an unknown woman. Discovering who she is and who killed her puts the purposeful Miss Butterworth in competition with Detective Ebenezer Gryce, still sleuthing at 77.
The solution is surprising and the characterization excellent. It's nice to see protagonists of these ages.