"A New England Nun" opens in the calm, pastoral setting of a New England town in summer. As the village settles in for the evening, the narrator introduces the main character: a young woman named Louisa Ellis. On this particular evening, Luisa sits quietly by herself in her home, sewing. She then prepares a beautiful tea, complete with fancy china, despite being the sole guest. After her tea, she feeds her dog, Caesar, and washes up before returning to her sewing. The narrator describes Louisa's actions with an air of routine and normalcy; it appears that these are Louisa's primary activities.
That evening, Joe Dagget visits. Joe and Louisa receive each other politely but strain to make conversation. Joe takes up space in Louisa's clean, orderly home, and knocks her possessions out of order. When he leaves, Louisa is secretly relieved. The narrator then goes back in time to reveal that Joe and Louisa have been engaged for fifteen years. For fourteen out of those fifteen, Joe was in Australia, making his fortune. During that time, the two barely spoke, and Louisa became so accustomed to living alone that she barely thought of her future marriage. Now that Joe is back in the village, however, their marriage soon approaches.
Louisa feels mild dread at the prospect of losing some of her precious domestic freedom. She fears needing to please Joe's elderly mother; giving up her idle hours spent distilling essences and mending linens for the pure fun of it; losing the ability to keep her house in perfect order; and freeing her elderly dog, Caesar, who she believes is fierce and dangerous. Joe also no longer loves Louisa as he once did, and he can sense that his presence in her house makes her uncomfortable. However, the two are bound by the forces of responsibility and respectability, and they passively accept their fate as future husband and wife.
One day, a week before the wedding, Louisa goes for a walk in the lush New England evening. Sitting quietly by herself on a stone wall, she hears the voices of Joe and Lily Dyer, the young woman who helps care for Joe's mother. Louisa listens to their conversation as Joe and Lily discuss their love for each other and the fact that they can never be together since Joe will never go back on his word to Louisa. Lily plans to leave the village to make things easier for both of them. Louisa is stunned by this conversation and stumbles home. When Joe Dagget comes to visit the following evening, she summons all of the diplomacy she possesses and manages to call off their engagement without ever once mentioning Lily Dyer. The two say goodbye with wistfulness and respect.
That night, Louisa weeps a little at the loss of her engagement, but the next day, she feels like the queen of her domestic paradise. Contented, she embarks on a life of orderly and pristine solitude.