Elinor Carlisle (ELISABETH DERMOT WALSH), a beautiful young socialite, stands on trial, seemingly unremorseful, being found guilty of double murder. Poirot (DAVID SUCHET) watches her from the gallery. All evidence points to her guilt, so why does he feel uneasy about it? Elinor thinks back to how it all started. It was such a happy time. She was in love and recently engaged, but then there was the letter... .. Elinor receives an anonymous letter that she shows to her fiance and cousin, Roddy Winter (RUPERT PENRY-JONES). The letter is about her Aunt Laura Welman (DIANA QUICK), who is also Roddy's Aunt through marriage. She has suffered from a stroke and is very ill. Elinor and Roddy stand to inherit thousands at her death. But the letter warns that 'someone is sucking up' to Aunt Laura, so that she and Roddy will get cut out of the will. And it warns that Aunt Laura will die of another stroke any day. Concerned about the letter and with a genuine care for Aunt Laura, the couple set off for the family home, Hunterbury. Both are surprised to find that Mary Gerrard (KELLY REILLY), the gardener's daughter, has also returned from her schooling in Germany. More beautiful than ever, she is looking after Aunt Laura, who obviously has a great fondness for Mary. Aunt Laura's doctor, the kindly Dr Lord (PAUL McGANN), consulted by Elinor, agrees to take the letter to his good friend and chess opponent, Poirot, who happens to be staying in the area. Poirot immediately has a bad feeling about the letter and deduces that the 'someone' in the anonymous letter is Mary Gerrard. Aunt Laura has another stroke. She asks for her solicitor to be sent for and tells Elinor that some provision must be made for Mary in her will. Much to Dr Lord's surprise, Laura dies in her sleep that night. They are further surprised to discover that Aunt Laura had never made a will and so as next of kin, Elinor inherits everything. The two nurses, Nurse O'Brien (MARION O'DWYER) and Nurse Hopkins (PHYLLIS LOGAN) had been taking it in turns to watch over Laura all night. Dr Lord decides not mention that the death could have been a mercy killing. Meanwhile, it has been difficult not to notice that Roddy has a definite attraction to Mary. On the night of Laura's death, Elinor catches them kissing. She confronts Roddy, who admits that he is falling in love with Mary. Elinor is devastated but maintains control and suggests Roddy takes a break in France and works out how he really feels. She returns her engagement ring with tears in her eyes and a newfound hatred for Mary. True to Mrs Welman's wishes, Elinor settles a generous amount of money on Mary. Mary accepts coolly, even announcing that with the nurses' advice, she had already made a will. Only Poirot seems to give Elinor any comfort about the whole episode and it is to him that she admits that she wishes Mary were dead! Elinor dismisses the servants for a week and then starts to pack up the house. As Mary and Nurse Hopkins clear out the gardener's cottage, Elinor invites them to the house for sandwiches. Elinor hands Mary a salmon paste sandwich. Nurse Hopkins makes tea for Mary and herself and joins them in the library. They have an uncomfortable lunch. Nurse Hopkins then returns to the kitchen to wash up. When Elinor arrives to help she finds Nurse Hopkins looking ill. Elinor notices a scratch on her arm. Hopkins says she caught it on a rose bush at the cottage. They return to the library to find Mary dead; poisoned with morphine. Elinor appears to have the motive and the opportunity to commit the murder; even Poirot has to admit the case against her is very strong, much to Dr Lord's dismay. Elinor is taken by the police. Laura Welman's body is exhumed and it is found that she too had died from an overdose of morphia. Elinor is convicted and sentenced to death. Poirot interviews everyone, trying to piece together a picture of Mary Gerrard. He spends sleepless nights thinking that something is wrong. Was it something that Nurse Hopkins said? He then realises that she was concealing something that could be important but he must work fast. Elinor only has five days before the sentence is carried out. But can he save her life?