After her mother left the family, Salamanca Tree Hiddle, better known as Sal, was forced to move from her family’s farm in Kentucky to Euclid, Ohio. Sal’s dad agreed not to sell the farm, but told Sal he needed to move away for a while as everything in Kentucky reminded him of her mother.
Sal spends a week in the car with her grandparents, and she keeps them entertained by telling stories about her friend Phoebe Winterbottom. Sal’s grandparents Gramps and Gram love hearing about Phoebe as they journey from Ohio to Lewiston, Idaho, where Sal hopes to find her mother. They will be following the path she traveled on a bus the previous year.
Phoebe lives across the street from Mrs. Cadaver, a friend of Sal’s father, and the reason they moved to Euclid. Sal does not like Mrs. Cadaver as she thinks she wants to take the place of her mother. Sal meets Phoebe in school, and the two become friends.
Like Sal, Phoebe is 13 years old. Unlike Sal, she is filled with worries and a vivid imagination. Phoebe also does not like Mrs. Cadaver. She thinks Mrs. Cadaver killed her first husband and planted him in the garden. Phoebe’s mother worries like her daughter. She also seems anxious to please her family, although they rarely show gratitude for her efforts.
One day, when Sal is over at Phoebe’s house, a young man comes to the door asking to see Phoebe’s mother. When told Mrs. Winterbottom is not home, he asks if Phoebe is her daughter. He repeats her name and then walks away. Phoebe is sure the young man is a lunatic.
Later that day, they find an envelope with a strange message about not judging people until you have walked two moons in their moccasins. Phoebe is convinced the note is from the lunatic. Phoebe gets several more random messages, but she and Sal can’t discover who leaves them. Sal and Phoebe have a friend named Mary Lou whose cousin, Ben, lives with her family. Ben often finds excuses to hang around Sal, and she finds herself attracted to him.
During the road trip, Gramps takes Sal on several diversions. At one, they swim in the Missouri River. A teenage boy approaches them with a knife. As he starts to steal Gramp’s wallet, Gram is bitten by a water moccasin. The boy helps to suck out the poison and directs them to the hospital. The boy stays the night in the waiting room with Sal and then leaves his name and address with her so she can write to him.
Gram insists on leaving the hospital to continue their journey. Sal remembers how her mother suffered a miscarriage and depression shortly before she left the family. Sal kept all the postcards her mother sent while on her bus trip and refused to believe her father when he said she would not be coming back to them.
Sal amuses her grandparents with more stories about Phoebe. She notices that Phoebe’s mother seems upset, but Phoebe does not seem to care. They also see the lunatic again, but run away before he can talk to them. A few days later, when Sal visits Phoebe’s house after school, she finds that Mrs. Winterbottom has left notes and frozen dinners for the family. Phoebe is convinced her mother has been taken by the lunatic, but Sal senses that Mrs. Winterbottom has left, just like her own mother. Phoebe searches the house for days, looking for clues as to who could have taken her mother.
While on their way to Yellowstone, Sal tells her grandparents how Phoebe went to the police, convinced her mother had been murdered and that Mrs. Cadaver should be a prime suspect. The police call Phoebe’s father to take her home. That night, Phoebe asks Sal to help her break into Mrs. Cadaver’s house while she is at work.
Phoebe wants to find evidence against her. Mrs. Partridge, Mrs. Cadaver’s blind mother, hears them and engages them in conversation. Before they leave, Mrs. Partridge comments that she recently met Phoebe’s brother. Phoebe tells her she does not have a brother. The following day, Sal is shocked to discover Mr. Birkway, their English teacher, is Mrs. Cadaver’s brother.
In class, arguments and embarrassment ensue when Mr. Birkway reads excerpts from the students’ journals. He changes names to try and protect the authors, but everyone knows who wrote what passage. He is upset when he reads Phoebe’s journal and learns she suspects his sister of murdering her husband.
That night, Mr. Birkway visits Phoebe and apologizes for reading her journal out loud. He explains that his sister’s husband was killed in a car accident, the same one that took his mother’s sight. Mrs. Cadaver was the nurse on duty at the hospital that night. Once Mr. Birkway leaves, Sal admits something else to Phoebe. She had seen a picture of the police sergeant’s son on his desk, and it was the lunatic.
Sal comes up with a plan to track down the lunatic. First, they call the sergeant, pretending to be a friend of his son’s. They learn that his son lives in a nearby town. On the weekend, they take a bus to look for him. Their schoolmate Ben is also riding the bus. Sal sits between him and Phoebe and cannot help how her heart beats fast whenever the movement of the bus causes them to touch.
He tells them he is on his way to the hospital to visit someone. When they get off the bus, Ben heads in one direction, while Sal and Phoebe set off for the university. They see Mrs. Winterbottom sitting on a bench with the lunatic. To their horror, she kisses the young man on the cheek.
Sal is too upset to watch anymore and flees down the road. She winds up at the hospital with Ben. He is visiting his mother who is in the psychiatric ward. Sal walks with him for a little while, and when she says she has to leave, she and Ben kiss for the first time.
Sal meets Phoebe at the bus stop. Phoebe is angry with her mother and seems not to care if she ever returns to the family. When they get to Phoebe’s house, her sister exclaims that their mother called and will be returning the following day, bringing somebody home with her. Phoebe refuses to help get the house ready for her mother’s return, but she begs Sal to come over to be a witness as she is too nervous to see her mother on her own.
When Mrs. Winterbottom arrives with the lunatic, whose name is Mike, she breaks down in tears. Mike is the son she gave up for adoption before she was married. She had not wanted her family to know of her past mistakes and had tried to live a respectable life. But Mike had found her, and she realized that her life was not respectable; it was small because she had not been honest with herself or her family.
After the initial shock, Mr. Winterbottom welcomes Mike into the family. Phoebe drags Sal outside where they find Mrs. Partridge leaving another secret note on the front porch. After learning she was the one that left all the notes, Phoebe returns to the house. Sal finally goes over to Mrs. Cadaver’s house to talk to her and find out why her father likes to be with her. When Sal returns home, Ben is waiting on her front steps with a gift. It is a chicken, because he knows she misses her farm. She gives him a kiss in gratitude.
As Gramps drives into Idaho, Gram becomes sick. They take her to the hospital. Gramps gives Sal the keys to the car and tells her to drive carefully to Lewiston. Although Sal has driven many times on the farm, she is nervous because it is night and the road is steep and narrow. She pulls over to search the side of a mountain.
A man stops to make sure she does not need help. He points out the spot where a bus went off the road the year before. He tells her that only one person survived the crash, but Sal already knows. She climbs down the mountain, hoping to find something of her mother’s in the wreckage, but it is an overgrown mess.
The sheriff is waiting for her when she climbs back up to the car. He is shocked to learn she drove herself up the mountain. He drives her back into town, to the cemetery where her mother is buried. While she sits at her mother’s grave, she hears a bird singing in a tree. Sal imagines it is her mother singing to her. She returns to the sheriff, convinced he is going to take her to jail.
Instead, the sheriff drives her back to Gramps. On the way, he tells her about finding Mrs. Cadaver in the wreckage. Sal knew this from her talk with her. Mrs. Cadaver had been sitting next to Sal’s mother on the bus. When Sal’s father had come to identify his wife’s body, he visited Mrs. Cadaver in the hospital, and they spoke about Sal’s mother. Mrs. Cadaver has been helping him to get over his grief.
When Sal and the sheriff arrive at the hospital, they find a note from Gramps explaining that Gram died in the night and where Sal could find him. Gram’s body is being flown back to the farm in Kentucky where Sal’s father will meet the plane. Gramps and Sal finish the arrangements and then head back home.
Sal realizes that Gramps needs to bring his wife’s body home to Kentucky so he can be close to her, but her father did not need to bring her mother’s body home because everything on the farm reminds them of her. Phoebe, Ben, Mrs. Cadaver and her mother are all planning to come out for a visit. Sal is excited to show them the farm and maybe have the opportunity to kiss Ben again.