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In the novel Murder in Mesopotamia, Mrs Marie Mercado is a young devoted wife, about 25, made uncomfortable by Louise Leidner. She is married to Mr Joseph Mercado.

Mrs Mercado is described as being "dark and slinky-looking". She has a "thin bird-like eager face with big eyes and rather a tight, suspicious mouth".

When Nurse Leatheran first arrives at the expedition house, she joins the rest of the party for tea, and finds Mrs Mercado staring at her with a kind of hungry stare. When Mr Mercado comes in, she transfers her attention to him, and watches him with an anxious impatience.

Mrs Mercado tells Nurse Leatheran about Louise Leidner's odd behaviour, such as reporting fingers tapping on her window, a hand without an arm attached, and a yellow face pressed against the window, and screaming when some shots were being fired in the village.

Shortly after that, she and Nurse Leatheran find Mr Mercado talking to Louise. Nurse Leatheran observes that Mrs Mercado looks "wild with fury".

Later in the novel, when Nurse Leatheran takes a bottle of acetone from the laboratory, Mrs Mercado confronts her, saying that Nurse Leatheran had known perfectly well that there was a bottle of acetone in the antika room, and that she went to the laboratory because she was spying on the expedition staff.

After Louise's death, Mrs Mercado sobs about how she was so fond of Louise. She tells Poirot that she was in her room shampooing her hair at the time of the murder, and knew nothing about it until she came out and was told. She later tells Poirot that all the members of the expedition were devoted to Louise, and that they were a thoroughly happy family, contrary to what Poirot had heard about the atmosphere of tension. Nurse Leatheran is of the opinion that this was all far removed from how Mrs Mercado actually felt about Louise, and Poirot agrees that she is "hardly the type to whom one would go for the truth".

During the denouement, Poirot says that Mrs Mercado is a "fiercely maternal type", and that she not only loves her husband, but would fight for him tooth and nail. He observed that she was constantly on her guard and uneasy. He explains that this uneasiness was for her husband, and not for herself. He suggests that there may have been a discreditable, or even criminal, incident in Mr Mercado's past, which Mrs Mercado had succeeded in covering up. If anything of this incident were to become known, it would ruin his career. Poirot considered it possible that Louise might have been able to induce Mr Mercado to confide in her, and that this would provide a motive for murder for the Mercados.


In the ITV 2001 TV adapatation of the novel, the part of Marie Mercado is played by Deborah Poplett. Like in the original, Marie disliked Louise Leidner. According to Amy Leatheran, judging by the way she looked at Louise, one would think she wanted to kill her. After Marie's husband committed suicide in a Baghdad hotel room, Marie was taken to Baghdad and she was not present at the denouement.