Sad Cypress is a 1940 murder mystery novel by Agatha Christie. The story is about Hercule Poirot's efforts to discover the truth behind the case of a young woman facing conviction for the murder of her ailing wealthy aunt and a lodge keeper's daughter who has become her aunt's companion.
Elinor Carlisle seems to be the only possible suspect for the murders of her wealthy aunt, Laura Welman, and her romantic rival, Mary Gerrard. The evidence was damning: only Elinor had the motive, the opportunity and the means to administer the fatal poison.

As Poirot begins to investigate, he discovers that almost everyone connected with the case is telling lies. However, Poirot knows that where crimes are concerned, a detective can learn as much from a lie as he can from the truth.