In Murder in the Mews, a young woman is found shot in a locked room. The police assume it's suicide, but Hercule Poirot begins to suspect murder.

Poirot and Inspector Japp must unravel the intelligent mind of Jane Plenderleith, Barbara Allen's housemate. Barbara was found in her room with a bullet lodged in the head.

The moment of death was disguised by the noise of fireworks. It turns out that Barbara had a love child when she was 17. The baby died but Major Eustace knows about it and is blackmailing her. When Barbara realizes that Eustace will eventually blackmail her fiance Charles as well, she kills herself.