A Mercy opens with the voice of the African slave girl Florens with a puzzling opening,
Don't be afraid....You can think what I tell you a confession,...but one full of curiosities familiar only in dreams and during those moments when a dog’s profile plays in the steam of a kettle.
She is on a journey to a blacksmith, her lover, carrying a message from her mistress, who is very ill. Florens alludes to her former home in Angola where Mr. Jacob Vaark arrived at the plantation of D'Ortega in order to collect a debt. However,it became apparent that D'Ortega could not pay his debt, so her mother offered Florens because she had seen how this man looked at her daughter. While Vaark does not want to take Florens, having been an orphan himself, he takes the girl.
The narration switches to a third-person and relates how Vaark has taken in a Native American girl, called Lina, who is the sole survivor of an outbreak of smallpox that has killed all the other members of her tribe. Also introduced are other members of the Vaark household, Messalina, or Lina as she is called, and Rebekka, Jacob's wife. The addition of an orphaned girl named Sorrow, a mixed-blood child from a shipwreck completes the odd family of orphans. As the fourth chapter ends, Lina wonders if Florens will complete her long journey to the blacksmith. The history of Rebekka is also told in this chapter. Sold off as a mail-order bride by her father, Rebekka considers herself fortunate to have Jacob Vaark as a husband. But, when her two children die, the boy in the construction of Vaark's grand house, Lina consoles her. As the chapter ends, Lina wonders if Florens will return in time with the blacksmith.
In the middle of the novel, Florens continues her journey and her thoughts are recorded in a stream-of-consciousness not unlike the style of William Faulkner in his novel As I Lay Dying,
What I read or cipher is useless now....But my way is clear after losing you who I am thinking always as my life and my security from harm, from any who look closely at me only to throw me away. From all those who believe they have claim and rule over me. I am nothing to you.
This stream-of-consciousness is continued in the third person in Chapter 7 as Rebekka lies ill and reflects upon her life. Then, in Chapter 10, Florens arrives at the blacksmith's. He tells Florens that he will travel alone because it will be faster; meanwhile Florens can care for the foundling that he has named Malaik. But, jealous of the attention that the boy takes from the blacksmith, Florens takes the boy's toy from his and tries to restrain him, accidentally breaking his arm. The blacksmith observes this and orders her to return to Rebekka, telling her she is a slave to lust, "a wilderness."
When Florens returns, the two indentured servants believe that they see Jacob's ghost. Without him, the family formed out of need and odd love seems to weaken and there is only a "claiming of self" as one critic writes. As Rebekka considers selling Florens and giving Sorrow, the girl who has an imaginary friend and is too naive to understand her pregnancies away, Sorrow wants to escape. But, Florens wants to finish her story to the blacksmith.
The novel ends with Florens's mother as narrator. She explains to Floren why she gave her up:
Because I saw the tall man see you as a human child, not pieces of eight....Hoping for a miracle.... It was not a miracle....It was a mercy. Offered by a human. I stayed on my knees