Third girl - Agatha Christie
Norma Restarick seeks help from Poirot, believing she may have committed murder. When she sees him in person, she flees, saying he is too old. He pursues the case finding that Ariadne Oliver sent Norma to him. He believes there is a murder that prompted Norma’s fears. Poirot and Mrs. Oliver gather information, visiting her parents’ home and her apartment building. Norma does not return home after a weekend visit to her father and stepmother. Mrs. Oliver finds her in a café by chance with her boyfriend David. Poirot meets Norma at the café, where she mentions the death again. After describing the odd times where she cannot recall what has happened she leaves in fear again. Mrs Oliver trails David, ending up in the hospital after being coshed on the head upon leaving his art studio. Poirot arranges for Dr Stillingfleet to follow Norma; he pulls her to safety from a close call with speeding traffic and brings her to his place for treatment and for safety.
Norma’s father Andrew abandoned her and her mother Grace when Norma was about 5 years old. Andrew had run off with a woman in a relationship that ended soon. He travelled in Africa in financially successful ventures. Norma lived with her mother until Grace’s death two and a half years before. Andrew returned to England after his brother Simon died a year earlier, to work in the family firm, arriving with a new young wife. Norma can recognize nothing familiar in this man, but accepts him. Norma is the third girl in her flat in the fashion of young women advertising for a third girl to share the rent. The main tenant is secretary to her newfound father; the other girl, Frances, travels often for the art gallery that employs her.
Mrs Oliver learns that a woman in the apartment building had recently died by falling from her window. A week passes before she tells Poirot, who feels this is what bothers Norma. The woman was Louise Charpentier. Norma says that her father ran off with Louise Birell. Later, Mrs Oliver finds a piece of paper linking Louise Charpentier to Andrew. Mary Restarick has been ill from poison in her food. Sir Roderick engages Poirot to find documents missing from his files which encounter brings young Sonia under suspicion.
Norma is lured from Dr Stillingfleet by an ad in the newspaper to meet David, and is again drugged. Frances kills David. She sets it up to appear that Norma did it, but the blood on the knife was congealed when Norma found herself holding it. With police and family gathered in the flat, Poirot announces that Andrew did die in Africa. Robert Orwell poses as her father to gain the wealth of the family. He had David paint portraits of him and his late wife in the style of a painter popular 20 years earlier as part of the ruse. Most cruelly he and his wife have been giving Norma various drugs that give her hallucinations and an altered sense of time, to set her up as guilty. Further, the wife had poisoned herself hoping to pin that on Norma, too. Louise wrote to Andrew on learning he was back in England, so Frances killed Louise; this is the murder Norma feared she did. The woman posing as stepmother was also Frances, who used a blonde wig to cover her dark hair when changing roles. Poirot takes the wig from her bag to make that point. Murder of the two who could expose the imposters was just one of her crimes. Sonia is exonerated when she finds the papers Sir Roderick misplaced, and the two will marry. Poirot had chosen Dr Stillingfleet to help him with Norma in hopes the two would marry, and they will.