The one-paragraph version: Nurse Lib Wright travel to Ireland on a two-week assignment to observe Anna O’Donnell, an 11-year-old devoutly religious girl whose parents claim she can live without food. Lib eventually realizes (with the help of William Byrne, a journalist) that Anna was being secretly given food before and is now starving to death. Moreover, Anna is choosing to “fast” and letting people believe it is a miracle because she considers it a sacrifice to save the soul of her recently-deceased brother, who had been molesting her for years. The family does not believe Anna’s claims (of molestation) and the local priest blames Anna for being molested. Lib finally decides to kidnap Anna and make it look like Anna’s remains were burnt up in a fire. The book ends with Lib and William traveling to Australia with Anna, who is eating again.
In Chapter One, Lib Wright is young woman who became a nurse, training under the famed Florence Nightingale, after her husband died. As the book opens, she is travelling to Ireland for a two-week assignment, where she learns that she is being tasked with “observing” a girl named Anna O’Donnell, an 11-year-old girl who claims to have abstained from food for the last four months. The girl says she no longer needs it, and a small committee has been formed to help prove or disprove these claims.
To do so, Lib and another nurse named Sister Michael will be switching off 8-hour shifts to watch over Anna at all times. The village Anna is from is run-down and poor. Lib is certain the O’Donnells are fraudsters, and she finds the O’Donnell family too backwards, too pious, too superstitious and poor. Lib finds it suspicious that the O’Donnells have a donation box — which they claim is handed over to the church — for travelers who wish to witness Anna’s “miracle”.
In Chapter Two, Anna proves to be a bright and devoutly religious girl. She is also very slight, suffers from fluid retention, is fairly weak and is easily winded. Lib also thinks she’s losing her hair a little.
Lib continues to scour the room for hiding places for food, and she asks Anna’s doctor, Mr. McBrearty, to restrict Anna’s visitors to prevent them from secretly bringing in food. An exception is made for Dr. Standish, chief of medicine at Dublin hospital, who handles Anna roughly, declares Anna to be in hysterics and orders Lib to force-feed her — though Lib refuses.
Meanwhile, the “spirit grocer” (liquor store) where Lib is staying doubles as a makeshift inn, and soon a journalist from the Irish Times, William Byrne, shows up. He is frustrated over being refused admittance to see Anna and is angry when he learns that Lib is the one who is prohibiting visitors.
As her shifts go by, Lib considers a number of possibilities for how Anna is managing this. Anna believes she is feeing off “mana from heaven”. Dr. McBrearty has a theory that Anna is living off light. Lib considers that someone may be sneaking Anna food, or that Anna is simply good at self-control and has genuinely started fasting now that they’re monitoring her.
In Chapter Three, Lib also learns that Anna had an older brother, Pat, who died suddenly nine months ago. Anna worries he is in hell or will be stuck in purgatory for a long time because he didn’t give a confession before he died.
Meanwhile, William Byrne continues to stay at the spirit grocer’s, and Lib confides in him about her thoughts on what is going on with Anna, though she continues to refuse him access to the girl. Their conversation gets increasingly free and touches on personal subjects, but when William continues to ask to see Anna, Lib suspects he’s using her to get to Anna.
In Chapter Four, Lib misses having someone to talk to and wonders if she misjudged William. She finally decides to arrange a meeting between Anna and William. Afterwards, William (who grew up in Ireland during the famine) tells Lib that Anna is wasting away. He’s seen famine before and her distended limbs and her vinegary breath indicate that her body is eating itself. Anna is also soon entirely bedridden and losing her teeth.
Once Lib realizes William is right, Lib tries to convince Sister Michael and Dr. McBrearty that Anna is simply a girl who is starving herself to death out of some combination of religious fervor and grief over her brother’s death. However, they both refuse to stop the situation. At William’s suggestion, Lib turns to trying to convince Anna to eat, but with no success. William learns that a few months ago, a group of missionaries had been in town for three weeks spreading the fear of god, culminated in a lengthy Blessed Sacrament that took place the day before Lib’s 11th birthday. It was then that Lib took communion, which also turned out to be her last meal.
Lib then figures out that Anna’s mother Rosaleen had been secretly feeding Anna by chewing up food and giving it to Anna by giving her a “kiss” at the morning and at night. Anna had genuinely believed it to be “manna” from heaven, but is now refusing that as well. Lib tells Anna the truth about the “manna” and that if she doesn’t eat it then she’ll die, but Anna simply smiles.
In Chapter Five, Lib learns that a certain prayer that she hears Anna repeat constantly everyday is supposed to help release souls from purgatory (if said 33 times a day while fasting on a Friday). In other words, Anna is fasting and praying in hopes of saving her brother’s soul.
That night, Lib and Sister Michael attends a committee meeting uninvited to tell them the truth about Anna’s health, bringing Anna in to show them her deteriorating state. Still, the committee decides to finish out the experiment just in case.
(Lib also confesses to William the truth about her background. She had an infant that died, and her husband left her after that happened — so she’s not really a widow. Her family distanced themselves from her after the scandal of her husband leaving her.)
Desperate to save Anna, Lib tries to understand why Anna is going to such extremes, and asks Anna if Pat did something bad. Finally, Anna admits that he began molesting her when she was 9 and he was 13. He had told her they were secretly “married”, but she later learned they’d committed a mortal sin. Anna told her mother about it after Pat died, but her mother said she was lying.
With Anna close to death and taking her last rites, Lib decides to concoct a plan to kidnap Anna. She convinces Anna to drink milk by saying that it is “holy milk” that will let Anna die and allow her to be reborn as another little girl. Anna agrees to imbibe the milk. When the family leaves the house for a special mass, Lib hands Anna over to William, who leaves with Anna. Lib then burns down the house. (She tells the committee it was an accident. They dock her pay, but do not charge her with any crime.)
In the Epilogue, Lib is now going by the name of Eliza Raitt. William is going by Wilkie Burns, with Anna pretending to be his daughter as Nan Burns. The book ends with Lib and William getting married by the ship’s chaplain as the three of them sail to Australia.
For more detail, see the full Chapter-by-Chapter Summary.