In the first chapter, Caleb asks Anna, his older sister, about his mother. Because his mother passed away the day after he was born, he asks all manner of small questions about the things she would say and the songs she would sing, because he wanted to know her. Papa returns from going to town, telling them that he has put out an ad in the newspaper in order to take applicants for a woman he can marry, and who can provide a mother figure for the children. A letter has come in response, from a woman named Sarah Wheaton who lives in Maine; she says that she wants to move on from taking care of her brother.
The family all write letters to Sarah individually, asking her their own questions to gauge their interest in her. Caleb asks about her opinions of smaller houses, and asks about her snoring. Anna asks about things like braiding hair, and asking about her favorite color. They receive letters back from Sarah; she says that she can braid hair, that she does not snore, and that she would like to bring her cat with her. The cat would stay for a month if the children would like; the family agrees and thinks it is a good idea. Sarah responds that she will come to them on the train, and to look for a woman wearing a yellow bonnet, and who is plain and tall.
It is now spring; Sarah has arrived by train. While Papa gets dressed up and goes off to pick her up, the children wait around at home, full of excitement. When Sarah arrives, she brings them both gifts from the sea, in order to fully connect them with where she came from. She also seems sad, and that she misses the sea; the children decide to be on their best behavior in order to convince her to stay. The children pine for a mother, and are fascinated by Sarah’s new experiences and excited to have a new parent figure.
In this chapter, Sarah talks about the conch shells the brought as gifts from the sea, looking sad while she listens to it, as she misses the sea. She also shows them the best ways to pick flowers and dry them by hanging them up. Caleb is forever insistent that Sarah will stay with them, and he frequently says it. Sarah and the children talk about the differences between the prairie and her home in Maine. Sarah sings a song, and Papa and the children join in; it becomes clearer that they are becoming a family, and the children grow to like Sarah.
Sarah is taken to see the livestock, which excites her, as she has never touched a sheep in her life. However, she found a dead lamb, which bring her great grief. She grows to adore all of the animals, telling the children about how Maine has a lot of dunes. In reply, Papa takes them all to the side of the barn, which has a ‘dune’ of its own, a giant hay pile that Papa made for her to make her feel more at home. Climbing the hay pile, Sarah laughs in glee and slides down it, with Papa and the children joining her. Sarah draws pictures for the children.
Sarah is taught by Papa to learn how to do field work, including plowing. Sarah inquires about how the prairie winters go; they respond that winter is when they go to school. Sarah seems to fear the winter, or at least be hesitant about enduring it. Talking to them, Sarah informs them that she is quite skilled in writing and math, saying that she loves to read. At one point, Sarah goes into the cow pond to take a swim, as she is hot, and she also wants a little bit of home. She dreams a “perfect” dream. Since the children had never swum before, Sarah decides to teach them how.
As prairie neighbors help each other out despite their distance, Matthew and Maggie arrive soon after to give Papa some help in plowing a field. They also bring some chickens over to give Papa’s family as a gift. Maggie’s situation is similar to Sarah, as they had both answered ads in the paper asking for a wife and mother; Maggie is from Tennessee, while Sarah is from Maine. The two women share stories about the things they miss about their homes; Anna overhears this and starts getting afraid that Sarah wants to move back to Maine. Maggie brings Sarah some flowers, and they both plant them, becoming fast friends. They decide to start learning from each other, and Maggie tells Sarah that, regardless of where you are, you will always miss something about the other place.
Sarah, being eager to learn how to do more things, dedicates herself to learning how to ride horseback and drive wagons. Her main motivation is to learn how to drive into town on the wagon, which Anna and Caleb take to mean that she might actually go to town, hop on a train and leave them forever. Despite this, Sarah dons some overalls and gives Jacob some assistance in fixing the house’s roof. This surprises Caleb, as women do not normally wear overalls in his experience. As this is happening, the family experiences a strong thunderstorm, which they counter by going into the barn after gathering all the animals into safety.
Once the storm passes, the family clears the damage from the storm and perform their usual chores. Sarah learns more about wagon driving. The following morning, she wakes up early in order to drive into town; Caleb notices this and schemes ways to make her stay, like tying her up or feigning illness. Knowing that she might never be coming back, Papa and the children spend the whole day being incredibly anxious about whether or not Sarah is coming back from town. Eventually, she comes back at dinnertime, the children being incredibly excited at the fact that she is going to stay. When Anna points out they were scared that they were all scared that she left, Sarah replies that she definitely plans to stay, and that she thinks of Papa’s place as her home now. Sarah presents them with some gifts that she bought for them while in town, and Anna mentions that she thinks that she and Papa will get married soon. The family has, by the end of the book, been fully formed, and they are all happy together.