The three-paragraph version: In December 1926, Agatha Christie goes missing for 11 days (resulting in a nation-wide search) after her husband Archie announces he’s leaving her for his mistress, Nan O’Dea. In a flashback to her younger years, we learn that Nan once fell in love with a boy in Ireland, Finbarr. She got pregnant and was sent to a convent where the nuns permitted her and another girl, Bess, to be sexually abused by the priest, Father Joseph. She was forced to give up her baby.
In 1929, Agatha ends up seeking out Nan during her disappearance, prompted by Finbarr who showed up trying to win Nan back. Agatha’s adopted daughter Teddy is likely the baby that Nan had taken away from her, which is why Nan has tried to seduce Archie. When two dead bodies (Mr. and Mrs. Marston) turn up at the hotel Nan is staying at, it turns out that Mr. and Mrs. Marston are actually one of the nuns from the convent (Sister Mary) and Father Joseph — they had decided to run away together. Bess and Nan devised a plot to get revenge/justice by murdering them.
Meanwhile, Agatha engages in a romance with Inspector Chilton, one of the inspectors tasked with finding her. Chilton covers for Nan’s crimes (saying that Mrs. Marston poisoned both her and her husband) once he understands what happened. The book ends with Nan choosing to marry Archie in order to be with Teddy, Finbarr returns to Ireland, and Agatha goes to be with Chilton.
The book is narrated by Nan O’Dea, the mistress of Archie Christie, and jumps back and forth from the days around disappearance of Agatha Christie and Nan’s younger years in England and Ireland.
In Part I, Nan urges Archie to finally leave his wife, Agatha Christie, who is a well-known writer. Agatha and Archie have a daughter, Teddy. Nan doesn’t love Archie, but she wants him anyway.Archie goes out to buy an engagement ring and the next morning he breaks the news to Agatha. Agatha is heartbroken.
The next morning, no one can find her and Agatha’s car is discovered hanging precariously off the edge of a chalk pit. A nation-wide search for Agatha begins. Archie tells Nan to stay away for the time being, and Nan goes North to Yorkshire to stay at the Bellefort Hotel and Spa. There, Nan comes across her former love, Finbarr Mahoney, who is looking for her. Meanwhile, Inspector Frank Chilton is one of the many men searching for Agatha and he also goes to stay at the same hotel. As he conducts his search, he happens upon Agatha staying at a manor house nearby.
Nan also narrates about her past in Ireland. She grew up in a family of girls in England and spent summers at her Uncle’s farm in Ireland. In Ireland, she meets and falls in love with Finbarr, a boy a few years older who helps out around her Uncle’s farm. When WWI starts, Finbarr enlists and goes off to war. During that time, Nan’s older sister Colleen gets pregnant, gets kicked out of the house by their father and kills herself.
When Finbarr returns from the war, he and Nan plan to get married, and they sleep together. Finbarr soon falls gravely ill from the flu, and Nan learns she is pregnant. Recalling what happened to Colleen, Nan leaves home and goes to find Finbarr’s family in Ireland. However, the Mahoneys refuse to take her in, and Finbarr is too weak to have a say. Instead, they drop Nan off at a convent.
In Part II, we learn that Agatha had been distraught and got into an accident. She had narrowly missed running into Finbarr, who had received a letter from Nan not too long ago describing her affair and her plan to seduce Archie. Finbarr was looking to talk to Agatha, since it seems they have a mutual interest in preventing Nan’s plot. Coincidentally, an elderly lady with dementia, Annabelle Oliver, had abandoned her car a short distance away from Agatha’s accident. Finbarr and Agatha took the abandoned car and drove off, ending up at the manor house where Chilton found Agatha (and Finbarr).
At the Bellefort Hotel (where Chilton and Nan are staying at), Nan meets the various guests including a number of couples on vacation. Tragedy strikes when one of the couples, Mr. and Mrs. Marston, is poisoned. Mr. Marston is found to have been injected with potassium cyanide, and his wife somehow ended up ingesting Strychnine later that night. An American couple, Lizzy Clarke and Donny, depart right after the murders. There’s also a young but very unhappy couple there, Mr. and Mrs. Race.
After finding Agatha and Finbarr at the manor house, Chilton permits Agatha a day before turning her in. When he returns the next day, she is gone. However, when Finbarr comes to see Nan at the hotel, Chilton recognizes him and is able to follow Finbarr to the new house that he and Agatha are staying (squatting) at. Chilton finds himself attracted to Agatha and unable to turn her in. He and Agatha kiss and eventually sleep together. Agatha reveals that Nan is pursuing Archie because she believes that Agatha’s (adopted) daughter Teddy is the daughter Nan had taken away from her many years ago.
Flashing back to Nan’s time at the convent, the pregnant girls there were put to work and not permitted to leave. They stayed until they gave birth and they were kept there to nurse the babies until they were adopted out. She soon learned the priest there, Father Joseph, was sexually abusing one of the girls, Bess. When Bess left, Father Joseph’s attentions turned to Nan.
When Nan finally gives birth, Nan names the baby Genevieve and hopes to find a way to escape with her. However, before she can, Genevieve is adopted away to an English family. Nan is furious and attacks Sister Mary Clare, who had pretended to be her friend while subjecting her to these horrors, but she doesn’t kill her. Nan then escapes the convent and goes home. She learns that Finbarr has been writing to her, not having been told what happened to her. When he finally arrives in England looking for her, Nan tells him about Genevieve and sends him away. One day, Nan spots Teddy and is certain that Teddy is her daughter Genevieve.
In Part III, Chilton and Agatha investigate the Marston murders and learn that Mrs. Marston was once a nun and Mr. Marston was a priest. They recently renounced their vows and married. Chilton and Agatha piece together that Mrs. Marson is Sister Mary Clare, Mr. Marston is Father Joseph and that Nan is likely the one that killed them.
We additionally learn that Bess and Nan were in on it together, both determined to get justice for their time at the convent. Lizzy and Donny Clarke were actually Bess and her husband, and Mr. and Mrs. Race were Bess’s sister and brother in law.
Agatha tries to convince Chilton not to arrest Nan and instead accept that justice was served. Chilton, Agatha, Nan and Finbarr spend a few more happy days at the manor house. Finally, various reports of sightings of Agatha in the area result in Archie showing up at the Bellefort Hotel. He’s relieved to find her, and Agatha claims she does not remember what happened. Chilton knowingly reports the false conclusion that Mrs. Marston must’ve poisoned her husband and then poisoned herself, thus letting Nan off the hook.
While Agatha leaves with Archie, their marriage soon ends Nan then marries Archie and they raise Teddy together (she chooses Teddy/Genevieve over Finbarr), and Finbarr returns to Ireland. (The book leaves it slightly ambiguous weather Teddy really is Genevieve.) As for Agatha, after her marriage ends, she goes looking for Chilton, and they embrace.
For more detail, see the full Section-by-Section Summary.