Toni Morrison's A Mercy is the story of a young slave girl named Florens. Florens is 16 years old and living on the D'Ortega, a plantation. The reader learns that the D'Ortegas spent four years in Angola, a place where the Portuguese were extremely cruel to their slaves. Among the slaves on the tobacco plantation are Minha mae and her children. The D'Ortegas have fallen on difficult times. Jacob Vaark, a trader, arrives at the plantation to collect a debt owed by D'Ortega.

It is clear that Vaark dislikes D'Ortega and practically everything he stands for from his cruel ways to politics, arrogance, and religious beliefs. Vaark can barely stand to be at the D'Ortega house at all and is glad to be able to leave. D'Ortega admits that he cannot pay the debt but offers one of his slaves as partial payment. Vaark presents himself as a more moral man than D'Ortega which impresses Minha mae. Minha mae decides to convince Vaark to take Florens as payment. Florens thinks she is being cast aside by her mother whose only goal is to protect her adolescent daughter.

Florens is sent to live on the Vaark farm with Jacob and Rebekka Vaark plus other slaves and servants. Life on the farm is surprisingly good compared to Florens' life at the D'Ortega plantation.

The story focuses on Florens but also gives the backgrounds on Lina, another slave, Jacob Vaark, Rebekka Vaark and Sorrow, another slave. Each has had trials and tribulations but when brought together form a type of family that is rare in that day and age.

Things change drastically when Vaark becomes ill. He becomes ornery and sullen, too weak to do anything but sleep. Vaark dies and sends the household into a frenzy. The panic increases with Rebekka's illness. The slaves like Rebekka and try to help as much as possible. Florens and Lina are sent off the property to find the blacksmith. Florens is thrilled to be the one going since she has developed an obsession for the man. Florens' opinion soon changes with the behaviors of the people on the outside. No one offers help, even those who know Lina. When help arrives it does so in the form of the seven Ney brothers, who clearly have ill intentions.

Florens spends a great deal of time making her way through the wilderness. At long last, Florens reaches the blacksmith and he goes to cure Rebekka. By this time, Rebekka is on the road to recovery. Upon arriving at the blacksmith's house, Florens receives a shock. The blacksmith is unmarried yet he seems to have created his own family, although it is supposedly temporary. The blacksmith has taken in a foundling, a young boy named Malaik. Florens and Malaik scuffle and the boy is injured. The blacksmith returns and there is a brutal battle between him and Florens. Florens is sent away.

Florens becomes despondent. Willard and Scully, the white indentured servants, make note of all the changes and wonder what will happen to their little family.

Florens begins to come to terms with her life, including past and present. The final chapter is written by Minha mae, who explains her actions and how above all she wanted her daughter to receive mercy.