Poirot’s triumphant return with Japp and Summerhaye to Styles signals the climax of his investigation. Gathering the household, he begins reveling in the details of his deductions. Mary Cavendish, under the respectful eye of Poirot, permits the assembly in the salon, hinting at the depth of her trust in the detective.

Poirot meticulously deconstructs the case, starting with the analysis of the bedroom where the tragedy unfolded. Key evidence—the fragment of green fabric, a stain of coffee on the carpet, and the mysterious box of bromide powders—leads him to establish Mrs. Cavendish’s presence in the room at the critical moment, challenging initial assumptions.

Through logical sequencing, Poirot unravels the mystery of the locked door, revealing Mrs. Cavendish’s unwitting role in the fabric of the crime. A careful investigation of Mrs. Inglethorp’s actions and statements prior to her death gives further credence to his theory, drawing attention to the strained familial relationships and hidden motives.

The alteration of Mrs. Inglethorp’s will is another puzzle piece, leading to the shocking revelation that Mrs. Inglethorp herself destroyed the new will amidst the chaos of her final evening. Poirot explicates the peculiar circumstances of the will’s destruction, citing the unlikely decision to light a fire as a desperate measure to dispose of the document—an act that initially went misunderstood.

Furthermore, Poirot shifts focus to the manner in which the poison was administered, debunking the theory of the coffee and pointing instead to Mrs. Inglethorp’s medication. This revelation underscores the cunning manipulation of everyday substances to carry out the murder, a detail overlooked by everyone except Poirot.

The denouement sees Poirot piecing together the final moments, clarifying the ill-fated mix-up of medications and the strategic planning behind the murder attempt. His deduction exposes not only the murder method but also captures the tumultuous relational dynamics within Styles, especially highlighting Mary Cavendish’s distress and innocence in the ordeal.

In a dramatic conclusion, Poirot presents physical evidence, sealing his case with irrefutable proof of the murderer’s identity and motive, concluding yet another masterful display of his detective prowess. His unraveling of the case lightens the burden of suspicion from the innocent, particularly Mary, and restores a semblance of order to the shattered household of Styles.