Mr. March, a Union chaplain during the American Civil War, writes home to his wife Margaret and their daughters, trying to reassure them with poetic descriptions of peaceful skies. In truth, he witnesses death and devastation constantly. Early in the war, he tries to save a wounded young soldier named Silas during a retreat but fails. Silas drowns in the Potomac, and March keeps a scrap of his coat as the only thing he can send to the boy’s mother.

While searching for a hospital, March finds himself in a former plantation house now used as a medical station. The sight triggers a flood of memories. As a young man, March visited the same estate as a traveling salesman. The owner, Mr. Clement, admired his intelligence and welcomed him to stay. March accepted, dazzled by Clement’s library and the chance to learn from a cultured man. He also met Grace, an enslaved woman whose eloquence surprised him. March began secretly teaching Grace’s niece Prudence to read, even though doing so was illegal. Grace supported the lessons, but Prudence’s mother, Annie, worried about the risk. Eventually, Clement discovered what was going on. Grace took the blame to protect Annie and Prudence. In response, Clement forced the entire plantation, including March, to witness Grace being violently whipped. March stood by, unable to stop the whipping. After, Clement expelled him, and March resumed his travels. Soon after, March began speaking out more forcefully against slavery.

March later became wealthy through his peddling business, settled in Concord, Massachusetts, and married Margaret. They shared a commitment to abolitionism and built a home where they raised their daughters. However, March lost everything after secretly donating nearly all of their savings to John Brown. He believed he was funding peaceful settlements for formerly enslaved people, but Brown used the money to plan a violent raid instead. When the plot collapsed, March covered Brown’s debts and confessed the truth to Margaret, who quietly endured their new life of poverty. When the war began, March volunteered as a chaplain, hoping to serve the cause and impress Margaret. Margaret supported the war but never wanted him to enlist.

Back in the present, March is still at the Clement planation, which is now a makeshift hospital. He discovers that Grace still lives there, caring for the elderly, senile Mr. Clement. She tells him that Clement is her father and that he sold her mother years earlier. Though Clement no longer recognizes her, Grace remains out of a sense of duty. March apologizes for his failure to defend her. She shows him the scars on her back and offers comfort instead of blame. Their bond deepens, and one night they embrace. A doctor sees them and reports March for impropriety, prompting his reassignment.

The army sends March to Oak Landing, a Southern plantation leased by Ethan Canning, a young Northern lawyer. There, March begins working with formerly enslaved people who still labor in cotton fields. He sets up a school and teaches children and adults to read and count. He meets Zannah, a mute woman, and her son Jimse. He also discovers the abuse they face: Canning punishes one man by locking him in a dry well, and many workers suffer from hunger and illness. March clashes with Canning over discipline and resource shortages, but eventually they collaborate on efforts to improve conditions.

The workers respond to March’s kindness with trust and cooperation. When supplies arrive from abolitionist donors—thanks in part to Margaret—morale rises. The community celebrates with a night of dance and chanting. March joins them and wakes the next morning with a severe fever. Canning seeks medical help, but the doctor offers only useless remedies. March recovers slowly. Then, news arrives that Union forces are pulling out of the region. Canning panics, fearing raids by Confederate forces or guerillas, but March urges him not to abandon the workers or hire armed guards, which would spark panic. They wait nervously.

Confederate guerrillas attack soon after. They search for March, who hides. When the raiders fail to find him, they execute an elderly man named Ptolemy and capture Canning. They burn the plantation and force the workers to march into the woods as captives. March, ashamed of hiding, joins Jesse, one of the workers, and trails the guerrillas. Jesse poisons their liquor, planning to act once they are weakened. When one of the guerrillas threatens to execute Canning, March breaks cover and intervenes. A violent fight erupts. Zannah frees the captives during the chaos. Canning dies protecting March. March takes a bullet, and Zannah pulls him to safety. Zannah carries March to Union lines and delivers him to a hospital ship. There, a nun tells him that Zannah left a message calling him a good man.

The story shifts to follow Margaret’s perspective. She travels to Washington after receiving news of March’s condition. She finds him filthy, near death, and neglected by staff. However, after Margaret complains, March gets a new nurse to look after him: Grace. When Margaret sees the level of intimacy that exists between Grace and her husband, she suspects an affair. Grace later details their shared past to Margaret, though she insists March does not love her and they have not had an affair. She also points out that a lock of Black hair Margaret found among March’s things likely came from a child he failed to save.

Margaret feels betrayed, but she pushes aside her anger to help March recover. At Grace’s urging, she talks to him about their daughters. As he improves, March announces that he cannot return home yet. He insists on redeeming himself and continuing to serve in the war effort. Margaret tries to convince him otherwise, but he remains fixated on atonement. While March is still recovering, Margaret receives a letter from home revealing that their youngest daughter, Beth, is sick with scarlet fever. She leaves March in Grace’s care, hoping that he will return when he is able.

When March and Grace are alone, Grace reveals that Clement’s son—her half-brother—died after attempting to rape her. She fought back and it resulted in the man’s death. She tells March she does not want forgiveness, only for him to understand that she knows what it means to sin and desire redemption. When March offers to stay and work with her, she refuses to let him. She tells him to go home—his daughters need a father, and the future must belong to Black people leading their own cause.

March finally returns home on Christmas. His family greets him with love, but he feels separate from them. He watches Beth play the piano and smiles, but inside he carries the memories of war, guilt, and failure. Though his body has returned home, part of him remains behind with the dead.