Set in Nigeria in the 1950s, "Marriage is a Private Affair" opens with a couple, living in the city of Lagos, having a conversation. Nene believes Nnaemeka should write to his father to tell him about their engagement. Nnaemeka explains that he will tell his father in person, adding that he does not expect that his father will look kindly upon their union, because Nene is not a member of the Ibo tribe. Further, the engagement was not an arranged marriage, which is the custom in Nnaemeka's father's village.
Nnaemeka leaves Nene's place and walks home thinking about the letter his father, Okeke, recently sent him. The letter outlines Okeke's plans for Nnaemeka to marry Ugoye, a girl Nnaemeka remembers from his childhood for her tendency to beat up boys. Okeke insists in the letter that she will be a good Christian wife.
Weeks later, Nnaemeka visits home and tells his father he cannot marry Ugoye because he does not love her. His father doesn't see this as a problem. Nnaemeka tries in vain to explain that marriage is different today, and Okeke insists that nothing about marriage has changed. Nnaemeka asks Okeke's forgiveness as he explains that he is engaged to Nene. Instead of erupting in anger, as Nnaemeka expects, Okeke goes quiet for the night. Before Nnaemeka leaves, Okeke says their engagement is Satan's work and vows that he will never meet Nene.
Okeke barely speaks to his son. Fellow villagers commiserate with the old man and recommend that he hire a herbalist to prepare Amalile, a medicine that women use to keep their cheating husbands faithful. Okeke rejects the idea as superstition. Six months later Nnaemeka receives a letter from his father, who has returned their wedding photograph, only he has cut it up to separate the couple. Nene cries at the sight of the mutilated photo. Nnaemeka assures her that his father will come around, but years pass with Okeke refusing to have anything to do with his son and his daughter-in-law.
Eventually, news of Nene and Nnaemeka's happy marriage travels back to Okeke's village. However, Okeke becomes known for flying into a rage when his son's name is mentioned, so nobody passes on the information. The strain of hardening his heart against his son nearly kills Okeke, but he perseveres.
One day Okeke receives a letter from Nene in which she tells him that his grandsons would like to meet him. She promises she will stay in Lagos. Having learned about his grandsons, Okeke feels his stubborn resolution falling in. Simultaneously, a rainstorm occurs, pelting his roof with large sharp drops of rain. Okeke pictures his grandsons standing in the rain, shut out from his house.
The story ends with Okeke having difficulty sleeping that night. He is full of remorse for having rejected his family, and he fears that he will not live long enough to make it up to them.
