The story unfolds with Sophia Leonides (Stefanie Martini), the granddaughter of the late Greek-British businessman Aristide Leonides (Gino Picciano), visiting private investigator Charles Hayward (Max Irons) in his office. Sophia is convinced that her grandfather's death was no accident and wishes for Charles to investigate the circumstances surrounding it. She reveals that Aristide’s regular insulin injection had been tampered with using a substance from his eye drops, ultimately leading to his fatal heart attack. Sophia's strong belief in foul play propels the reluctant Charles into the case, especially since they share a romantic history from their time in Cairo.
With permission from Chief Inspector Taverner (Terence Stamp) of Scotland Yard—who shares a connection with Charles through his father—Charles begins his investigation at the sprawling Leonides estate. As he interviews various family members, he uncovers layers of resentment and potential motives for each. The atmosphere is charged with tension, as all family members stood to gain from Aristide's wealth, yet harbored ill feelings towards his domineering personality.
Among the family, Lady Edith de Haviland (Glenn Close), sister of Aristide's deceased first wife, despises him for his affecting her life and the lives of her late sister’s children. She roams the grounds with a shotgun, venting her bitterness while considering Aristide a parvenu, given his immigrant background.
Then there’s Philip Leonides ([Julian Sands]), Aristide’s elder son, who struggled with his father's perceived neglect of him, coupled with his gambling addiction. His wife, Magda (Gillian Anderson), a fading theatre actress, feels his pain as they deal with their children Eustace ([Preston Nyman]) and Josephine ([Honor Kneafsey]), each dealing with their grandfather's oppressive nature in their own ways. Josephine, particularly, harbors resentment after Aristide cut her ballet lessons short, voicing her anger through subterfuge as she secretly spies on family activities.
Roger Leonides ([Christian McKay]), Aristide’s younger son, struggles to keep his family's business afloat, relying on bailouts from his father. His poisonous wife Clemency ([Amanda Abbington]) possesses an alarming level of knowledge about toxins, further complicating the web of deceit surrounding Aristide’s death.
Sophia’s suspicions about Brenda Leonides ([Christina Hendricks]), Aristide’s much younger second wife, intensify, especially when it comes to light that Brenda administered the fatal injections. Involved with Laurence Brown ([John Heffernan]), the children’s tutor who has been writing memoirs about Aristide, Brenda becomes a prime suspect, especially amid Roger’s fierce accusations.
As the investigation unfolds, a curious turn reveals an unsigned will, leaving Brenda poised to inherit everything. Tensions escalate as Charles learns of the family’s tangled histories and lies, particularly regarding Aristide's unsavory past involving CIA dealings.
Sophia’s disturbing act of burning the only copy of Mr. Brown’s manuscript, along with the revelation of Josephine's dangerous encounters, only further embroils Charles in his quest for the truth. When Josephine's life hangs in the balance after a suspicious accident, suspicions extend even towards Sophia, complicating Charles's feelings and determination to uncover the truth.
Ultimately, tensions culminate in a shocking revelation: Josephine meticulously orchestrated the entire string of deaths. Her motives stem from her bitter rejection and her despair over her controlled life. As Lady Edith sacrifices herself, jumping from the edge of a quarry to free Josephine from repercussions, the intimate and tragic ramifications of the family’s dark secrets leave Charles holding a grieving Sophia at the precipice of the cliff, struggling to process the layered complexities of love, betrayal, and tragedy.
