In the climax of the story, however, she is revealed to be the killer of Paul Renauld, and she only wants to marry Jack so that she can obtain his father's fortune.
She was voiced by Francesca Buller in the 1990 radio adaptation produced by BBC Radio 4, and by Sophie Linfield in the 1996 television film adaptation from Agatha Christie's Poirot.
Marthe was only nine years old when her mother murdered her father with the assistance of Paul Renauld. Growing up, Marthe had inherited her mother's manipulative and cunning side. After discovering Paul Renauld as her neighbor, Marthe began to get closer to Paul's son, Jack, and began to date him. Her true purpose was to take over Renauld's family fortune through her marriage with Jack, and their relationship had worried Renauld and his wife.
One day, Marthe overheard Renauld's plan to fake his kidnapping and death in order to escape from Madame Daubreuil. At night, she followed Renauld out and found him digging grave for the corpse of a tramp, who died in his garden by accident. She stabbed Renauld dead and pushed his corpse into the hole.
After Jack was threw into prison for suspicion over killing his father, Bella Duveen who was in love with him had falsely took the blame of killing Renauld. However, Poirot revealed that Marthe was the real killer based on the information he had gathered during the investigation. Together, he and Eloise Renauld staged up a ploy to expose Marthe as the culprit.
Following Poirot's strategy, Eloise pretended to openly disinherit Jack, who left her alone in the villa (under Poirot's arrangement), giving Marthe a chance to attack Eloise at the night in order to gain the fortne once and for all. However, Marthe was killed by Dulcie Duveen (Bella's twin sister, and later the wife of Arthur Hastings) in time, saving Eloise's life and exposing the truth.
- In Agatha Christie's Poirot, before attacking Eloise, Marthe was stopped by Poirot and Hastings, and was killed by Eloise's secretary. The character Dulcie was merged with her sister into a single character, with Bella married Hastings in the endings instead of Dulcie. In the original novel, Bella married Jack, and Dulcie married Hastings.