Third Girl is a 1966 novel by Agatha Christie.
Hercule Poirot is enjoying a quiet breakfast when a strange young woman shows up unannounced at his door. The disheveled, nervous young woman says that she was referred to Poirot, and that she may have committed a murder. No sooner has she gotten that out, however, than she takes a second look at Poirot, decides "you're too old", and departs.
Poirot, with his vanity offended and also curious, starts asking around. Luckily, his friend Ariadne Oliver (a mystery writer and Agatha Christie's Author Avatar) knows who she is, because Ariadne was the one who referred the young lady—one Norma Restarick—to Poirot. Poirot then finds out about Norma's whole social circle: her father Andrew Restarick, once estranged from Norma but now returned after a couple decades abroad; Andrew's young wife Mary; Norma's roommates Claudia and Frances; and David Baker, Norma's long-haired hippie boyfriend who comes off as extremely suspicious.
Poirot and Ariadne find all this out, but at the same time Norma herself seems to have disappeared. Where is she? Did she commit a murder?
- Author Avatar: Another appearance of Ariadne Oliver, a fussy old English lady with a Motor Mouth who writes detective novels and has a Funny Foreigner protagonist she dislikes. In other words, Agatha Christie.
- Chekhov's Gun: The numbers to the apartments in Norma's building are affixed with simple nails that sometimes fall out. Frances used this to trick Norma into thinking she was in front of Louise's apartment and might have been responsible for Louise's death.
- Comic-Book Time: Played with. Poirot was said to be retiring from detective work some forty years before this book, at the time of Christie's earliest novels, so it's curious that he's still taking jobs in the 1960s. On the other hand, he is also specifically described as "old" in this book and out of touch with hip Sixties London.
- Continuity Nod:
- In The Clocks, Poirot says he's writing a book about detective fiction. In the first chapter of this novel he's contemplating his completed book; Poirot doesn't like Edgar Allan Poe and Wilkie Collins.
- When tricking Sir Roderick into thinking they were war colleagues, Poirot name-drops Colonel Race (a character from Death on the Nile and Cards on the Table), and Inspector Giraud (from The Murder on the Links).
- Dr. Stillingfleet previously appeared in Christie short story "The Dream".
- Dead Person Impersonation: The Reveal. It turns out that Andrew Restarick is dead, and the "Andrew Restarick" who engaged Poirot's services is actually Robert Orwell, an impersonator who hopes to claim Restarick's vast fortune.
- Disappeared Dad: Andrew Restarick left his wife and daughter when Norma was five, and spent some fifteen years abroad. This turns out to be crucial to the solution.
- Foreshadowing: Poirot is able to make his way into Sir Roderick Horsefield's confidence by convincing Sir Roderick that he, Poirot, was an old spy colleague of Sir Roderick's during World War II. This idea, that people can easily fool the doddering old man about his past, is referred to again at the end when "Andrew Restarick" is revealed to actually be an impostor, whom his great uncle Sir Roderick failed to spot.
- Funny Foreigner: Poirot says "I hope I do not derange you" to Mrs. Restarick, a bit of deliberate Obfuscating Stupidity that he uses from time to time throughout the novels.
- Gaslighting: Andrew/Robert and Mary/Frances have been conducting quite a cruel campaign against poor Norma, drugging her up and gaslighting her and planting things on her, like a bloodstained knife, to convince her that she is a murderer and insane.
- The Generation Gap: A running theme of the book has old coots like Poirot and Ariadne Oliver continually boggling at the young people of 1960s swinging London. In the first chapter Poirot evaluates Norma Restarick's short skirt, high boots, and unkempt hair: "She wore what were presumably the chosen clothes of her generation....Anyone of Poirots age and generation would have had only one desire—to trop the girl into a bath as soon as possible." Mrs. Oliver nicknames David Baker "the Peacock" for his long curly hair and flashy dress, although Poirot notes that long hair and frilly cuffs and collars are Older Than They Think and that David would look perfectly at home in Regency-era England.
- Incredibly Obvious Tail: Ariadne catches Norma and David in a restaurant together and decides on a whim to tail David. She tails him on a long and twisty journey through Carnaby Street, until he finally turns and confronts her.
- It's for a Book: When David catches Ariadne Oliver on her Incredibly Obvious Tail, Ariadne says that she was following him for her books, that she was studying how easy or hard it is to tail a person.
- Lady Drunk: Louise Charpentier, the "suicide" in Norma's apartment building. She was in her mid-forties and reportedly depressed about it, and she drank heavily.
- The Matchmaker: Poirot is revealed to have gotten Dr. Stillingfleet, The Shrink, together with Norma, not just for him to give her psychotherapy but also in the hopes that they would fall in love. They do.
- Never Suicide: Poirot is shocked to find out that a woman in Norma's apartment building, Louise Charpentier, jumped out her window to her death. Naturally, it was murder, and connected to the mysterious plot around Norma.
- Quizzical Tilt: Poirot's reaction to the news that Sir Roderick Horsefield has shown up at his home unannounced."He considered this with his head on one side, looking rather like a robin..."
- Scatterbrained Senior: Sir Roderick Horsefield, an addle-minded old man whom Poirot is easily able to convince that Poirot was an old acquaintance of his During the War. He's also addle-minded enough to not know that "Andrew Restarick" is an impostor.
- Sexy Secretary: Sonia, the gorgeous young secretary/assistant to doddering old Sir Roderick Horsfield, Andrew's uncle. Sir Roderick is smitten with her.
- The Shrink: Dr. Stillingfleet, whom Poirot sends to Norma to give her some help and get her out of the clutches of whoever has been drugging her up.
- Sinister Switchblade: Some teenagers had a flight with "flick-knives" in the courtyard of the apartment building where Norma lives. Frances said she found a flick-knife that appeared to be stained with blood in Norma's drawer.
- Summation Gathering: All the main characters wind up together at the scene of David's murder, which is where Poirot makes The Reveal—by revealing that all the characters are together, instead of one being missing like everyone thinks.
- Title Drop: Norma is frequently referred to as the "third girl", this being an arrangement where two young women sharing an apartment look for a third woman to share the space.
- Totally Radical: Christie mostly avoids Totally Radical awkwardness in her portrayal of 1960s hippies by, after all, making her POV characters out-of-touch old coots like herself. But she does fall into this trope with her drug references, like when a character talks about "sniffing snow" and when friendly Dr. Stillingfleet discusses what's in Norma's bloodstream:Dr. Stillingfleet: Id say shed been taking purple hearts and dream bombs, and probably L.S.D.
- Two Aliases, One Character: It turns out that Mary Restarick and Frances Cary are the same person. Mary/Frances wears wigs and uses makeup to disguise her identity, and it helps that basically only one person, the jittery, drug-addled Norma, knows them both.
- Wig, Dress, Accent: Frances Cary, the bohemian, dark-haired art gallery salesgirl, puts on a blonde wig and dresses more respectably to masquerade as Mary Restarick.