Mrs Oliver learns that a woman in the apartment building had recently died by falling from her window. The main tenant, Claudia, is secretary to her newfound father the other girl, Frances, travels often for the art gallery that employs her. Norma is the third girl in her flat in the fashion of young women advertising for a third girl to share the rent. Norma can recognize nothing familiar in this man, but accepts him. Andrew returned to England after his brother Simon died a year earlier, to work in the family firm, arriving with a new young wife, Mary. Norma lived with her mother until Grace's death two and a half years before. He travelled in Africa in financially successful ventures. Andrew had run off with a woman in a relationship that ended soon after. Norma's father Andrew abandoned her and her mother Grace when Norma was about 5 years old. 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. Mrs Oliver trails David, ending up in the hospital after being coshed on the head upon leaving his art studio. After describing the odd times where she cannot recall what has happened she leaves in fear again. Poirot meets Norma at the café, where she mentions the death again.

Mrs Oliver finds her in a café by chance with her boyfriend David. Norma does not return home after a weekend visit to her father and stepmother. Poirot and Mrs Oliver gather information, visiting her parents’ home and her apartment building.

He believes there is a murder that prompted Norma's fears. He pursues the case finding that Ariadne Oliver sent Norma to him. When she sees him in person, she flees, saying he is too old. Norma Restarick seeks help from Poirot, believing she may have committed murder. It is uncommon in that the investigation includes discovering the first crime, which happens comparatively late in the novel. The novel is notable for being the first in many years in which Poirot is present from beginning to end.

It features her Belgian detective Hercule Poirot and the recurring character Ariadne Oliver. The UK edition retailed at eighteen shillings (18/-) and the US edition at $4.50. Third Girl is a work of detective fiction by Agatha Christie and first published in the UK by the Collins Crime Club in November 1966 and in the US by Dodd, Mead and Company the following year.

Third Girl by Agatha Christie