Salamanca Hiddle is a thirteen year old girl who tells both her own story and that of her friend Phoebe Winterbottom. Sal has moved away from the childhood home that she loves after her mother left; her mother is now peaceful in Idaho, and unable to bear the constant reminders of his wife, Sal's father, JohnHiddle, moves them to Euclid, a town that makes Sal feel constrained and claustrophobic. Her father also spends a great deal of time with a woman called Margaret Cadaver; they have been getting to know each other by letter and telephone and it was Margaret who found John his new job in a nearby office. Sal wants nothing to do with Margaret. As her mother's birthday approaches, Sal's grandparents take her on their road trip across the country to visit her mom in Idaho. Her grandparents will see many of the tourist spots they are excited to visit and Sal will get to see her mother.
Sal's grandmother asks her to tell them a story as they drive. Sal decides to tell them about Phoebe, her over-dramatic best friend whose mother suddenly vanished one day. Phoebe invited Sal to her house for dinner after being impressed by Sal's bravery in catching a spider and removing it from the classroom. By coincidence Phoebe lives next door to Margaret Cadaver. Phoebe suspects that Margaret is a lunatic who murdered her husband. Phoebe's family are polite and proper and obsessed about lowering their cholesterol. Mr Winterbottom is a cartographer and Mrs Winterbottom is a home-maker but Sal has a feeling that she doesn't actually enjoy this role. One Saturday Sal was at Phoebe's house when the doorbell rang and the girls saw a young man of about eighteen standing outside. They answer the door and he asks if it is the Winterbottom home. Phoebe asks if he would like to leave her mother a message but he declines. Phoebe is convinced he will return and ambush them so they head out to visit their friend Mary Lou. Her home is the opposite of Phoebe's as everyone is wild, happy and spontaneous. Sal is immediately drawn to Mary Lou's cousin, Ben, who lives with Mary Lou's family.
Phoebe's mother decides that they should not mention the young man at the door to Phoebe's father. Phoebe finds an anonymous note at the door that says "Don't judge a man until you've walked two moons in his moccasins". The arrival of the note spooks everyone and nobody seems to know why it might have been left.
Sal's Gram is starting to feel sorry for Mrs Winterbottom but nonetheless presses Sal to continue. By now the Hiddle road trip has reached Madison, Wisconsin, eating at Ella's Kosher Deli and Ice Cream Parlor, followed by lemon tea and blueberry muffins at the Steep and Brew. They then visit the Wisconsin Dells where they visit an old fort and see American Indians dancing and playing drums in a circle. Sal has always been told to call Indians "Native Americans" but her mother, who was descended from the Seneca Indian tribe, hated the term and thought American Indian far more noble a name.
Leaving Madison, they head through the lower rim of Minnesota where Gram wants to hear more about Phoebe.Sal resumes the tale and reveals that Phoebe received more messages; the next read "Everyone has his own agenda." Phoebe's conviction that all strangers are potential axe murderers is starting to rub off on Sal as when they are buying candy at the drugstore she feels someone way hung them; looking around, she sees the you g man who visited Phoebe's house looking for her mother.Ben walks the girls home and Phoebe shows her mother the second note.
Gram and Gramps are caught up in the story and as they detour to Pipestone they discuss what the cryptic sayings might mean. They visit Pipestone National Monument and smoke peace pipes and stay overnight at the Injun Joe Peace Palace Motel. The next day Sal tells Gram and Gramps about Mr Birkway. Sal suspects he is not all there because he is excitable for a teacher. He has given a summer assignment to the class to keep a journal; everyone is horrified when he wants to read them. Sal was not at the school when the assignment was given so he allows her to just write one or two sample entries instead. One Saturday they notice him in Margaret Cadaver's backyard where he and Mrs Cadaver dig around a rhododendron tree; Phoebe suspects that is where Margaret buried her husband's body. Neither girl thinks Sal's father should visit her anymore.
In Sioux Falls, the weather is scorchingly hot so Gramps pulls off the highway and he and Gram go skinny dipping. Sal joins them. Her hair floats all around her in the water and reminds her of her mothers. They see a boy on the shore going through Gramps' pants pocket. Sal is frightened of him but before she can react Gram flails in the water and realizes she has been bitten by a poisonous snake, a water moccasin. Gramps swims her back to the riverbank and the boy lends Gramps his knife so that he can make a slit in Gram's leg to let out the snake venom. The boy sucks out the wound and accompanies then to the hospital where Gramps insists on lying in the hospital bed with her. She is a difficult patient and consequently released from hospital the next day. They continue driving towards the Badlands and Sal resumes her story.
Phoebe arrived home from school to find a note from her mother telling her to keep all the doors and windows locked and to call her father if she needs anything. This didn't sound that worrisome until Phoebe's sister Prudence opened her note which asked her to boil spaghetti and heat up the sauce. Their father's note stayed she had to leave for a few days but offers no explanation. Phoebe believes her mom has been kidnapped and wants to call the police but her father observes a lunatic kidnapper would not have allowed her to label the food in the freezer for them and write notes. Phoebe tells her school friends that her mother is on a trip in London. At home she searches for clues, finding in identifiable spots on the carpet that she marks with tape, and collecting some unknown hair strands in an envelope. Feeling sorry for Phoebe, Sal invites her to spend the weekend at her house. Phoebe accepts but is a difficult and demanding guest. She hopes her mother will have returned by Monday but she has not. She has apparently called Mrs Cadaver to let her know all is well but Phoebe finds this very suspicious and thinks Mrs Cadaver has murdered her mother and has made up the story about the telephone call to hide this.
Phoebe went to the police herself, Sal in row. The police did not take their suspicions seriously and called Phoebe's father to pick the girls up from the police station. Phoebe is upset because nobody takes her fears seriously. Sal begins to prepare Phoebe for the reality that her mother might have left for good but Phoebe won't let her talk this way. On their second visit to Sergeant Bickle, Sal notices that the lunatic young man who keeps showing up at Phoebe's house is in a couple of the photographs on the police officer's desk. It appears he is their son.
Back at Phoebe's house, Mr Birkway is again visiting and Sal discovers that he is Margaret Cadaver's twin brother. She is therefore anxious when he reads Phoebe's journal as she has written about her fears that Mrs Cadaver, a suspected husband-murderer, has done away with her mother as well. Mr Birkway visits Phoebe at home and tells her that although Margaret's husband is dead, she did not murder him. He was killed by a drunk driver and her mother was blinded and crippled in the accident. Margaret had been a nurse in the emergency room when they were brought in. Phoebe feels very guilty for getting her scenario so wrong but is still baffled as to her mothers whereabouts. When Sal tells her the lunatic is Sergeant Bickle's son, they devise a plan to track him down.
Sal breaks her story when she and her grandparents arrive at Old Faithful, a natural geyser that still bubbles and erupts. It is an incredible sight and brings Gram to tears as she has always wanted to see it in person. As they whiz through Montana, Sal resumes the story. Sal calls every Bickle in the telephone book until she finds the correct ones, and asks to speak with their son. They give her his number at school and they visit the university campus on Saturday. At the bus stop they run into Ben who is going to visit someone in hospital. At Chanting Falls university they head to Mike Bickle's dormitory and after claiming to be his cousins are directed to his door. On the way, Sal spots Mike sitting on a bench holding hands with Phoebe's mother. Phoebe watches them, angry, but Sal cannot watch and runs as far as possible the opposite way. Finding herself at the hospital she asks to see Mrs Finney but cannot go up to her ward as she is not family. She is directed to the garden where she sees Ben sitting with a woman wearing a pink robe. It is his mother and Sal is reminded of her own mother who was in a similar mental state after losing her baby. The attraction between Sal and Ben reaches a climax and they finally kiss each other. Returning to the university Sal finds Phoebe at the bus stop. Phoebe is furious that her mother has cut her hair and changed the way she does her make-up. When they get home, Prudence tells them that their mother called and she is coming home. Phoebe makes Sal come over for support.
Mrs Winterbottom is glad to see her husband but has brought Mike with her to introduce to the family. She admits that she had never told her husband for fear of what he might think of her, but Mike is her son, and she had given him up for adoption before she even met her husband. Sal leaves, realizing this is a situation she should not be involved in. Phoebe leaves with her and as they go they find Mrs Partridge placing a cryptic note on the step.
Gram interrupts the story by announcing they are in Idaho. They press on to Coeur d'Alene to stay the night before going to Lewiston where Sal's mother is, in the morning. Gram is tired but wants to hear what happened to Phoebe. She also wants to know why Mrs Partridge had been leaving notes on the Winterbottom's porch. This was something Sal and Phoebe also wanted to know, so they asked her. She had actually viewed them as nice surprises, not as potentially ominous and frightening, and she chose them from the sayings that Margaret read out of the newspaper.
Ben was sitting on her front step when she got home. He has brought her a chicken and named it Blackberry.
Gram is disappointed that the story has ended. She seems to be getting weaker so when they get to Coeur d'Alene they go directly to the hospital where they learn she has had a stroke. She is unconscious but Gramps insists on staying with her because they have been side by side for fifty-one years. Sal feels guilty and thinks the snake bite triggered the stroke. Gramps gives her the car keys and some money, knowing that she still wants to go to Lewiston. Sal can already drive, having learned on their farm. She drives slowly but determinedly and in four hours arrives at the top of Lewiston Hill. Halfway down she finds an overlook where a man points out the one thing she is looking for; the top of the bus that skidded off the road eighteen months ago in an accident that left only one person alive. Salcrawls down to the bus, wanting to look for anything familiar. When she returns to the car there is another car parked behind hers; when the sheriff hears her story he drives her to Longwood, to the Snake River, so that she can visit her mother's grave. She hears birdsong, and knows that the tree is singing.
The sheriff drives her back to Coeur d'Alene and they talk about the bus accident. He doesn't know if he saw her mother but remembers seeing Mrs Cadaver, who had been sitting in the seat next to Sal's mother and was the only survivor of the crash. It was not until Sal saw the bus and the gravesite that she had truly believes her mother was dead.
Back at the hospital her grandfather has left a note letting them know he is at the hotel and that Gram passed away.
Sal and her father go back to Bybanks, on their farm with Sal's grandfather living with them. Ben and Phoebe write to her all the time and are planning a visit. Sal is hoping for more kisses.