In 1925 Caroline Crale (RACHAEL STIRLING) is hanged for the murder of her husband, the famous bohemian painter, Amyas Crale (AIDAN GILLEN). Their daughter Lucy is sent away to Canada, the awful truth hidden from her. Thirteen years later Lucy (AIMEE MULLINS), now a sophisticated twenty-one year old, approaches Poirot (DAVID SUCHET), brandishing a letter that her mother wrote on the eve of her death. It claims that she is innocent. Poirot agrees to dig up the past and find out the truth, warning Lucy that it may not be what she wants to hear. Poirot acquaints himself with the facts of the trial and realises that there were five people present at this house that fateful summer of 1926. He interviews them individually and with great cunning and flexing of his little grey cells, he pieces together snatches of misinterpreted conversation and draws out secrets and telling letters that those involved hardly thought were relevant. The crime was thought to be a crime of passion. Amyas made no secrets that he had affairs but Caroline always knew that he would come back to her. This time it was different. Elsa Greer (JULIE COX) was a beautiful, bold eighteen-year-old and Amyas appeared infatuated. He brought her to the family house, Alderbury, and spent hours painting her against the backdrop of the water garden. It was his finest painting to date and he would stop at nothing to finish it. Tensions began to rise between Amyas, his wife and his mistress until the cruel Elsa blurts out that Amyas and she want to get married. Amyas can't deny it. Caroline is heart broken. The guests, including Philip Blake (TOBY STEPHENS), who is Amyas's best friend, Philip's brother Meredith (MARC WARREN), who owns the house across the creek from Alderbury and Miss Williams (GEMMA JONES), the governess, are all shocked and acutely embarrassed. Only the children, naughty Angela (TALULAH RILEY) Caroline's younger half sister and little Lucy, remain oblivious to the dark cloud hanging over the household. After the group has paid a visit to his amateur laboratory, Meredith realises that a dangerous poison is missing. After lunch, he discovers the use that it was put to. Amyas is found dead in front of his precious painting of Elsa. All the evidence points to Caroline and she is arrested and charged. Poirot then invites the main suspects back once more to Alderbury, which is now filled with fourteen years of dust and memories and prepares for a grand denouement. Was Caroline Crale guilty? The world's greatest detective recreates the events of that Summer revealing the intrigue, love and jealousies of those trapped in the past.