Elinor visits her aunt who complains about the fact that she is partially paralyzed from a stroke. She wants to end her life but her doctor will not hear of it. In the meantime, Roddy falls in love with Mary, prompting Elinor to end their engagement. After a second stroke, Mrs. Welma asks Elinor to make provision for Mary. However, before the will can be changed, she dies and her estate goes directly to Elinor.
Elinor sells the house she inherited and gives two thousand pounds to Mary. The latter dies of poisoning during a lunch at Hunterbury. Everyone at the house has access to the poison. Elinor is arrested. Later, everyone learns that Mrs. Welman also died of poisoning. Peter Lord, in love with Elinor, brings Hercule Poirot into the case. Poirot soons discovers the author of the letter. Was the poison in the sandwiches made by Elinor or in the tea prepared by Nurse Hopkins? Also, what is the secret of Mary's birth? What is the significance of the scratch made by a rose thorn on Hopkins' wrist?
It turns out Nurse Hopkins is the murderer. The thorn scratch on Nurse Hopkin's wrist is really an injfection mark from the needle full of emetic she injected herself with causing her to vomit up the poison in the tea. She went to wash the dishes so that no one would see her vomit. Mrs. Welman, and Sir Lewis Rycroft, had an illegitimate daughter and she is Mary. If this infomration had been learned sooner, Mary would have inherited some of the estate. When someone encourages Mary to write a will, she names her aunt, Mary Riley, from Australia as beneficiary. Mary Riley's married name is Draper. It turns out Mary Draper is really Nurse Hopkins, who is bent on getting her hands on the money.
Elinor is acquitted and she married Peter Lord.
Sad Cypress, published in 1939, courtesy http://www.agathachristie.com/stories/sad-cypress