Elinor Carlisle and Roddy Welman are the model English couple, perfect companions set for a life of ease when they inherit Aunt Laura's considerable fortune. But a poison pen letter begins a chain of events which is to end in tragedy. Convinced that Mary Gerrard, a childhood playmate of Elinor's, is attempting to ingratiate herself with her aunt for financial gain, the pair travel to the family home to investigate. They find no evidence but Roddy falls desperately in love with the beautiful Mary, little realising that beneath Elinor's restrained and unemotional exterior lies an almost obsessive passion for him. Elinor obeys her aunt's deathbed wish despite her heartbreak, and gives Mary a large bequest from the estate. But when Mary is found poisoned, the evidence against Elinor is damning. It's up to Hercule Poirot to find out if the case is as simple as it seems...