Anna, who narrates this tale, deeply misses her late mother and struggles to handle her troublesome younger brother. Caleb, a talkative child, also longs for the mother he never met. His endless questions annoy Anna, and he understands that his craving for attention can sometimes trouble others. When Sarah arrives, he worries he might scare her away.

Their father, Jacob Witting, is a solitary and diligent man. He mourns his wife just like his children but understands that he must eventually move forward with his life. To do so, he places a newspaper ad seeking "help." "You mean a housekeeper?" Anna inquires. "No," her father responds. "Not a housekeeper. A wife." He receives a response from Sarah Wheaton, who resides in Maine. Although she cherishes her life by the sea, Sarah agrees to Papa's offer to relocate inland and live with him and his children.

The narrative revolves around Sarah and her new family getting acquainted. Sarah is under no obligation to marry Papa and is free to leave if she wishes; much of the story's tension revolves around whether she will choose to stay. In a letter to Papa, Sarah describes herself as "plain and tall." Upon her arrival at the farm, she demonstrates practical wisdom, a willingness to help with even the most arduous tasks, and a gentle, warm demeanor.

One of the book's central themes is sharing. Through sharing, Sarah gradually becomes part of the family, and it's through sharing that the children come to know and love her. Initially, the children are uncertain about opening up to Sarah. Caleb fears his unruly nature might drive her away, so he keeps his distance. Anna worries that Sarah will miss the ocean too much and leave the farm to return to New England, making her hesitant to fully give her love to Sarah. Only the farm animals offer unconditional affection, unafraid of being hurt; as MacLachlan notes, "the dogs loved Sarah first."

Sarah's commitment to participating in all the farm's tasks, such as fixing the roof, is balanced by her desire for independence. When she learns to drive the wagon alone and heads into town, the children and their father worry she might not come back. However, upon her return, she not only asserts her freedom to come and go but also gains the trust of her new family. She demonstrates that her choice to remain is driven by desire, not necessity. MacLachlan skillfully portrays the evolving relationship between Sarah and her new family, creating suspense around Anna's fear of rejection while illustrating the gradual and believable development of bonds among the characters. This approach makes "Sarah, Plain and Tall" a poignant story about people discovering a sense of belonging and confidence in their own worth.