The Third-Floor Flat: a Hercule Poirot Short Story

A woman's body is found in the third-floor flat at Friars Mansion. And who should live in the very same building but Hercule Poirot! Another woman accidently locked herself out of her flat and her friends found an interesting way to get in and retrieve the key. But they made a mistake in their calculation as to which flat was hers. What did they find in the dark? Blood and a body. Our perspicacious detective then offered assistance.