Poirot watches Hastings read a letter from Dulcie Duveen. Dulcie writes that she couldn’t prevent Bella from confessing, admits to deceiving Hastings, and asks his forgiveness.

The letter traces the sisters’ involvement from the start. When Dulcie met Hastings on the Paris train, she was already alarmed. Bella had become convinced Jack had fallen in love with someone else and was determined to confront him at Merlinville. When Bella missed their planned meeting the following day and news of the murder broke, Dulcie feared Bella had killed Renauld in a fit of rage. Traveling to Merlinville, she was taken to view the body and recognized the familiar dagger beside it. Fearing Bella’s fingerprints were on the weapon, she faked a fainting spell, stole the dagger, fled to Calais, and dropped it into the Channel mid-crossing. Back in London, she found Bella in a wretched state, and they took a stage engagement to stay occupied.

When Dulcie spotted Poirot and Hastings watching their performance, she discovered that Hastings believed her to be Bella. Rather than risk that he wouldn’t extend the same protection to the real Bella, she maintained the deception. Once Jack was arrested, Bella refused to wait any longer. Dulcie signs the letter with her real name, having crossed out “Cinderella.”

Hastings asks Poirot if he knew the truth all along. Poirot confirms he did because the sisters, though nearly identical, were distinguishable. He admits he kept Hastings in the dark partly from wounded pride and partly to test the sincerity of his feelings. Hastings worries that the letter says nothing explicit about Dulcie’s feelings for him, but Poirot insists her affection is clear. When Hastings notes there is no return address, Poirot promises he can locate her easily.

After his release, Jack arrives with Stonor to see Poirot and Hastings. He blames himself for his father’s death due to his callous treatment of Bella and for the accident of wearing the wrong overcoat that night. At Poirot’s request, he recounts what he actually witnessed that night. Arriving at Merlinville by train, he took a shortcut across the golf links and heard a faint, frightening cry. Rounding a bush, he found his father face down in the open grave with a dagger in his back. Looking up, he saw Bella staring at him in horror before she fled. Unwilling to testify against her, he made his way back to Cherbourg.

A telegram announces that Mrs. Renauld has regained consciousness. Poirot, Hastings, and Jack leave for Merlinville immediately while Stonor stays to assist Bella’s defense. Jack goes to Marthe at the Villa Marguerite while Poirot calls on Mrs. Renauld. When Jack and Marthe arrive shortly after, Mrs. Renauld descends the staircase. She declares that she holds Jack morally responsible for his father’s death, vows to disinherit him, and describes Marthe as the daughter of his father’s worst enemy.

That evening, Poirot and Hastings find Dulcie Duveen waiting at the Hôtel de Bains under an alias Poirot arranged. She hands Poirot a second dagger, which she retrieved from Bella’s belongings at his request. This shocks Hastings, who believed the dagger had been thrown into the sea. Despite Poirot’s resistance, Dulcie insists on joining them.

The three stop at the Villa Marguerite to check on Jack, who is sleeping fitfully with Marthe watching over him, before concealing themselves in the bushes outside the Villa Geneviève. They hear a cry of distress from a room Poirot did not anticipate. Poirot hammers on the front door, then scales a tree and enters through an open window. Hastings and Dulcie follow, only to find themselves locked in an empty room. Dulcie, a professional acrobat, climbs back out and traverses the roofline to reach the lighted window, where she confronts the attacker. In the ensuing struggle, the assailant falls and strikes her head. A maid comes to the aid of Poirot and Hastings. They discover Mrs. Renauld is alive but badly shaken after nearly being strangled. Beside her lies a silk rope ladder and a motionless figure. Hastings uncovers the face and sees that it’s Marthe, who died from striking her head on a marble fender.

In the aftermath, Poirot rebukes the servants for failing to tell him that Mrs. Renauld had moved bedrooms after the original crimes. He sends Hastings for a doctor and the police and appoints Dulcie to stay with Mrs. Renauld.

The following morning, Poirot lays out his case. Marthe Daubreuil was the murderer. Bella made a false confession to save the man she loves. Both she and Jack arrived at the grave nearly simultaneously, and each assumed the other was responsible. The crime required detailed knowledge of Renauld’s staged escape plan, and Marthe, having admitted to overhearing a confrontation between Renauld and the unhoused man, could just as easily have overheard the full scheme. Her motive was clear: Renauld’s death would make Jack heir to a fortune and remove the main obstacle to their marriage. Jack had made three souvenir daggers—one for his mother, one for Bella, and one for Marthe. If the dagger Dulcie threw into the Channel was Marthe’s, then Bella would still have her dagger. Poirot had secretly instructed Dulcie to search Bella’s belongings. When she produced Bella’s dagger, his theory gained proof.

To force Marthe into action, Poirot arranged for Mrs. Renauld to publicly disinherit Jack, creating an urgent threat to Marthe’s ambitions. Marthe slipped out of the Villa Marguerite while Poirot spoke to her mother in the hall. Madame Daubreuil then took up her daughter’s position at the upstairs window, leading Poirot and the others to mistake her silhouette for her daughter’s. Marthe’s plan was to chloroform Mrs. Renauld and administer a fatal morphine injection, making the death appear self-inflicted. Poirot notes Hautet would likely have accepted this story. Finding Mrs. Renauld awake, Marthe resorted to strangling her and tried to escape via the silk ladder before Dulcie stopped her. Poirot adds that he first grew uneasy about Marthe on the very first day. Her visibly anxious expression puzzled him because she had no reason yet to worry about Jack.

In the days that follow, Jack recovers. Madame Daubreuil vanishes before the police can find her. Giraud suffers a mental health crisis and returns to Paris. Poirot tells Hastings he believes Jack’s feeling for Marthe was infatuation, not genuine love, and that Bella is where his real attachment lies. When Jack is well enough, Poirot tells him the truth about Marthe. Mrs. Renauld, at Poirot’s urging, then discloses that Jack’s father was the fugitive Georges Conneau. Poirot reassures Jack that, having worked for Renauld rather than the police, he has no obligation to expose his client’s identity.

Back in London, Poirot places a model foxhound on his mantelpiece, which he won on the wager with Giraud and pointedly named after his rival. Jack visits to announce he is immigrating to South America to manage his late father’s holdings, bringing his mother and Stonor. Poirot urges Jack to ask Bella to go with him, arguing that both proved their love through their willingness to sacrifice their lives for each other. The novel ends with Hastings proposing to Dulcie in the garden of the Villa Geneviève. She warns him she can only ever be herself, and he silences the objection with a kiss.