Emily Boynton, Piper Laurie the stepmother to Lennox Boynton, Raymond Boynton, and Carol Boynton and the mother to Ginevra, drives a careful and ruthless plan to destroy her late husband’s second will. That will would have split the fortune more evenly between Emily and the children, but she wants to keep the entire estate for herself, avoiding any share for her stepchildren. To pull this off, she convinces Jefferson Cope, the family’s lawyer, to help overturn the will and thereby maintain her control over the money and their lives.
She moves the family and Nadine, her daughter-in-law, on a European holiday to give herself a semblance of normalcy while she works to erase the second will. The journey begins with an air of tense politeness as the adults maneuver around Emily’s constant bullying and her determination to keep the youngsters in line. In Trieste, the gathering intersects with the famous detective Hercule Poirot, Peter Ustinov, who stumbles upon an old friend, Dr. Sarah King Jenny Seagrove. Dr. King’s arrival adds a romantic undercurrent, as she soon finds herself drawn to Raymond Boynton, which further unsettles Emily.
On board, Lady Westholme, Lauren Bacall, a recently naturalized British MP with American roots, enters the social mix along with Miss Quinton, the archaeologist Hayley Mills, and the wily Cope, who are all en route to Jerusalem and the surrounding sites of interest. The adult stepchildren uncover the existence of a second will that their father had hinted at before his death, a revelation that unsettles the household just as Emily’s domineering grip tightens. Nadine, the brain and heart of the love triangle that forms around Cope, seems to accept his courtship with a mix of calculation and affection, even as Emily pushes him to stay away from the family.
Emily’s malignant scheming escalates when she poisons Cope’s wine with a digitalis dose from her own medical supplies, intending to frame the death as a natural heart failure. The plan is foiled, however, when Nadine’s husband strikes Cope and reveals an engraved cigarette case Cope had given Nadine, causing the poison to spill. Poirot, ever observant, notes cockroaches drinking from the spill and dying, a macabre clue that reinforces his suspicion that more lurks beneath the surface than natural causes.
As the group arrives at an archaeological dig, Cope, Nadine, Lennox, Carol, Raymond, and Dr. King embark on a walk, but Lennox, upset by his wife’s preference for Cope, withdraws early. The others return one by one, and Dr. King notices an Arab man attempting to rouse Emily. When she investigates, she discovers Emily dead. Dr. King initially identifies heart failure, but Poirot warns that death in a family this large and deeply disliked is never straightforward. A closer look at Emily’s medical bag reveals a disordered setup, an empty digitalis bottle, and a missing syringe, suggesting an outsider’s involvement rather than a simple accident.
Poirot plans to question a local child who reportedly witnessed the murder, but the boy disappears, frightened away by an unseen third party. Dr. King pursues the boy through the streets, narrowly avoiding danger as a gunshot rings out and the boy is killed. Accusations swirl around Dr. King, but Poirot clears her to join the others for a planned “picnic” where he intends to reveal the truth. By suggesting that all the stepchildren lied about seeing their stepmother alive, Poirot shreds their alibis and turns suspicion away from them.
That evening, a banquet becomes the stage for the final revelation: Lady Westholme is the murderer. Emily had recognized Westholme from her years in prison, where she had once been a warden. To keep quiet and preserve her social standing, Westholme used digitalis from Dr. King’s bag to incapacitate Emily and silenced the witness who could have testified against her. The shock of the confession unsettles Westholme deeply, and she retreats to her hotel room in a torn state of guilt and fear.
The night ends with a fireworks display, and Westholme shoots herself rather than face a return to prison and further disgrace. Poirot persuades the local authorities to treat her death as an accident, avoiding any further tarnish to her reputation and the memory of Emily Boynton’s late husband. The truth stands, and the danger of old resentments and tightly kept secrets within a family finally yields to Poirot’s careful deductions and the decisive, if tragic, moral reckoning.
