In Agatha Christie's most twisted tale, a spy-turned-private-detective is lured by his former lover to catch her grandfather's murderer before Scotland Yard exposes dark family secrets.

In this classic Agatha Christie detective story, former diplomat Charles Hayward has returned from Cairo to London to become a private detective. When Aristide Leonides, a wealthy and ruthless tycoon, is poisoned in his own bed, Detective Hayward is invited to solve the crime. As the investigation deepens he must confront the shocking realization that one of the key suspects is Aristede's beautiful granddaughter, his employer and former lover; and must keep a clear head to navigate the sultry Sophia and the rest of her hostile family.

Multi-millionaire Aristide Leonides has just died, apparently of a heart attack. Private investigator Charles Hayward is approached by his granddaughter Sophia and asked to investigate his death as she believes he was murdered. Hayward takes on the case and visits the Leonides estate, questioning the family. He discovers that it is far from a simple case - the family is incredibly dysfunctional and nothing is as it seems.—grantss

When a wealthy tycoon is poisoned in his own bed, Detective Charles Hayward (Irons) is invited to solve the gruesome crime. He is shocked to discover that one of the key suspects is the tycoon's beautiful granddaughter Sophia (Martini), his former lover. Hayward must keep a clear head to navigate the sultry Sophia and the rest of her hostile family in a case where nobody is above suspicion.

The granddaughter of late Greek-British business tycoon Aristide Leonides (Gino Picciano), Sophia Leonides (Stefanie Martini), visits private investigator Charles Hayward (Max Irons) in his office. She wants Charles to investigate Aristide's death, and suspects he was murdered by a member of their sprawling and idiosyncratic family. Sophia notes that Aristide's regular insulin injection had been laced with Eserine from his eye drops, causing a fatal heart attack. Sophia believes this was deliberate, not accidental. Charles reluctantly takes on the case, in part because he had a brief love affair with her in Cairo. Charles seeks the consent of Chief Inspector Taverner (Terence Stamp) of Scotland Yard to look into the case, utilizing his personal connection with Taverner, who had served with Charles's father, a decorated former Assistant Commissioner.

At the Leonides estate, Charles interviews the various members of the family, finding motives for each of them. All of them get substantial bequests from Aristide's estate but all resented the way he bullied and manipulated them. Lady Edith De Haviland (Glenn Close) was the sister of Aristide's late first wife; she moved in to care for her motherless nephews. She despised her brother-in-law as a parvenu (at the time of the marriage, Aristide was an immigrant with no prospects and her sister was the daughter of an Earl) and for his callousness towards his grandchildren. Edith stalks around the grounds, blasting moles in the lawn with a shotgun. Aristide's elder son, Philip (Julian Sands), hated his father for passing him over as successor to the family business (He was addicted to Poker), and for refusing to fund production of a screenplay he wrote for his wife, Magda (Gillian Anderson), a fading theatre actress.

Philip and Magda provided Aristide with three grandchildren: Sophia, Eustace (a teenager crippled by polio) (Preston Nyman), and Josephine (a clever 12-year-old, who spies on everyone and writes it down in her notebook) (Honor Kneafsey). Josephine admits that she hated Aristide, as he stopped her ballet classes, stating that she is no good. The younger son, Roger (Christian McKay), is managing director of a major family business, but is a failure, requiring multiple bailouts from Aristide. His domineering wife Clemency (Amanda Abbington) is a plant biologist with extensive knowledge of poisons.

Aristide's second wife, Brenda (Christina Hendricks), is much younger, a former Las Vegas casino dancer. The others suspect her of killing Aristide, especially Roger, who denounces her as a gold-digging slut. She did administer the fatal insulin injection. Also, she is having an affair with Laurence Brown (John Heffernan), private tutor for the Leonides children. Brown was writing Aristide's memoirs. Brenda feels that the entire family was mooching off Aristide, and she could never get rid of any one of them. And when Aristide's will is produced, it is unsigned. Thus, he died intestate, and Brenda will inherit his entire estate. Meanwhile Roger admits to Charles that his company was in trouble. The night of the murder, he had a heated argument with Aristide, who wrote Roger a check for $ 2 MM. But Clemency asked him not to cash the check.

An added angle is that Aristide used to work for the CIA and funded anti-communist organizations in Greece, in exchange for help in covering up his shady businesses in US. Taverner asks Charles to get to the bottom of the mystery at the earliest as the stakes are too high, and international.

Charles finds Sophia burning the only original copy of the memoir's manuscript written by Mr Brown. Sophia explains that in WW II, Aristide was involved in making bombs that destroyed cities and then took fat Govt contracts to rebuild the same cities. She didn't want this part of his life to come out.

Charles's inquiries meet with hostility from most of the family. Josephine hints that she has found clues she does not disclose, to Charles's irritation. Events take a new, horrific turn when the ladder to Josephine's tree-house is sabotaged and she falls from the tree, having to be hospitalized. Charles suspects that this was due to Josephine's habit of spying on the other family members and the killer thus wanting to silence her.

Charles' suspicions even extend to Sophia after a new, properly signed will is discovered, leaving the estate to her. Eustace even suggests that Sophia hired Charles to investigate the murder due to their personal history, knowing he would never accuse her due to their romantic past. After those developments, Taverner arrives in person to take charge of the case; he feels Charles' romantic history with Sophia does not make him objective enough to solve it. The discovery of love letters between Brenda and Laurence gives Taverner enough evidence to arrest them for Aristide's murder and the attempt on Josephine.

Charles, however, remains unconvinced that Brenda and Laurence are guilty, noting Brenda's childlike intelligence and Laurence's pacifist, left-wing views as making them unlikely candidates for being murderers. Sophia and Edith seem to agree. Sophia notes that the letters could have been forged, and Edith insists on "the best lawyer" to represent Brenda and Laurence. Edith visits a London doctor, and learns she is dying of cancer. Charles returns to the estate after being in London when Josephine's nanny is fatally poisoned by hot chocolate that she had prepared for herself and Josephine.

Charles implores Josephine to name the killer, as he has worked out that she knows. Again, Josephine smugly refuses to tell, even when Charles warns her that she is in danger. Edith collects Josephine and drives out of the estate (supposedly for ice cream), lying to get past the police at the gate.

The coroner finds that the nanny (Jenny Galloway) died of cyanide poisoning, Charles suspects Edith, who used cyanide to kill moles. He searches Edith's garden shed, and finds a bottle of cyanide; also, Josephine's missing notebook, buried in quicklime that would have destroyed it. Charles and Sophia take off in pursuit of Edith, who left a note for Charles to find in his car. Sophia reads Edith's note: it is a confession to the murder, but Charles doesn't believe it.

Sophia then starts reading from Josephine's notebook and discovers the horrible truth: Josephine murdered Aristide. He had stopped her ballet lessons (with insulting comments), and she was bored. Josephine also staged her fall from the tree-house, poisoned the nanny, who had begun to suspect her, and forged Brenda's love letters. Lady Edith had worked out that Josephine was the killer and confessed to exonerate Brenda and Laurence, and perhaps to spare Josephine a life in psychiatric institutions and a lifetime reputation as a monster. As Charles and Sophia catch up, Edith drives off the edge of a quarry, killing herself and Josephine. The film ends as Charles comforts a shocked and sobbing Sophia at the edge of the cliff.