Seven-year-old Sam lives with her mother, Courtney, and two-year-old half-brother, Noah, in a small, unfinished house owned by Noah's paternal grandparents. Sam's father, Mitchell, a traveling magician and street performer, lives nearby but is often absent. Restless and physical, Sam climbs doorframes barefoot to entertain herself. When Mitchell takes her to the Topsfield Fair, she discovers a climbing tower and scales it in cold rain, slamming the bell at the top. The outing reveals Mitchell at his best and worst: adoring and spontaneous, but so unreliable that a stranger has to jump-start his car.
Mitchell introduces Sam to climbing at the local YMCA, where his friend Kevin belays her, managing her safety rope while she climbs a small wall. He calls himself Sam's coach but soon leaves for Boston. Sam and Mitchell share a private language of metaphors; her favorite is "my dad is my umbrella" (11). Courtney, exhausted from raising two children largely alone, calls Mitchell "unrealistic" (38).
When Noah's father, Jack, a volatile Gulf War veteran, moves back into the house, danger follows. Left alone with Sam while she is home sick, Jack throws her out the front door and locks it after she confronts him for smoking indoors. Sam tells Mitchell, and at a Halloween party, Mitchell attacks Jack on sight. The two fight viciously until bystanders separate them. Everyone blames Mitchell, and Sam feels responsible for telling. Mitchell disappears for weeks, then calls five days late for Sam's eighth birthday to announce he is moving to Portsmouth with a girlfriend. He gives Sam climbing shoes that are too big, promising "Room to grow!" (40).
Mitchell's promises go unfulfilled; Courtney reports he has no money and no girlfriend. After another violent confrontation between Jack and Courtney, Sam launches herself at Jack. Courtney grabs Noah, and they flee, eventually settling in a two-bedroom apartment. Courtney takes a second job, and Sam begins attending the YMCA afterschool program, where Kevin coaches climbing.
At the Y, Sam meets Halle, a tall, fearless reader whose father drives the girls to Boulders, a climbing gym in Newburyport. An instructor named Toby recognizes their potential and recommends them for the youth team. Sam earns a scholarship. She is faster and more creative on the wall; Halle is stronger and more methodical.
Sam enters her first competition weighed down by nerves. Mitchell promises to attend, but Sam asks him not to come, afraid he will distract her. She climbs stiffly and finishes poorly. Afterward, she accuses him: "You never try for anything" (71). He replies, "I'm trying now" (71). Over the following months, Mitchell stabilizes: He sells his performance equipment, takes a job at a garden center, and attends recovery meetings. At a spring competition with both parents watching, Sam hauls herself over the top of a grueling bouldering problem, a short rope-free climbing challenge, to secure a victory. She and Mitchell grow closer, walking the beach and drawing a map of imaginary literary places. Courtney observes that with Mitchell, "boring is exactly where you want to be" (84).
Stability fractures when Halle leaves for Andover, a boarding school. Sam loses her climbing partner and her rides to the gym. High school overwhelms her. A new coach named Declan, fierce and charismatic, replaces Toby and challenges Sam to prove she cares. Sam becomes intensely drawn to him. She dates a classmate named Corey but feels little attraction.
When Mitchell loses his license for driving under the influence, Sam begins riding the bus to practice. Declan offers her rides home. In the car one day, his finger traces her neck and collarbone; Sam is shaken but unsure whether it happened. Then Mitchell disappears again, promising to attend Sam's October meet but failing to show. Sam climbs with cold precision because she no longer cares, winning first in bouldering and fourth in lead, a category of roped climbing on taller walls. Courtney tells Sam that Mitchell is sick and that "some things can't be fixed" (128). Sam visits his group home but is told he is not there.
Declan moves Sam out of his coaching group. After a tense practice, she runs to the bus stop; he finds her, and the encounter becomes sexual. Sam is sixteen. They meet secretly at his apartment, where he whispers, "You were never here" (155). Sam tells him she is in love; he says he is confused about her. When Courtney catches Sam getting out of Declan's car, Sam promises it will not happen again but continues lying. One Saturday, Sam finds Declan and Toby in a hallway embrace and realizes he lied about not having a girlfriend.
Sam quits Boulders and refuses to explain why. Courtney, assuming the withdrawal stems from grief over Mitchell, arranges a visit. Mitchell tells Sam he is caring for rescue horses at Windy Hill Farm, performs a card trick, and promises to let Sam set the pace. After he leaves, she finds the red queen in her back pocket.
Sam takes Noah hiking at Red Rocks, a conservation area in Gloucester. They get lost in the dark but find their way back. When they arrive home, Courtney is waiting: Mitchell is dead of an overdose. Sam drives to the beach and sits alone, recognizing that dead means too late for everything. A memorial is held at Windy Hill. Sam walks through the rest of senior year in silence, telling no one about her father's death.
On prom night, Sam drives to Gloucester and stumbles upon outdoor climbers at Red Rocks, including Justin, a twenty-four-year-old gardener in a battered brown fedora. She borrows shoes and climbs a route faster than he does. Over the following weeks, she climbs regularly with the group. Justin shows her a massive tipped boulder no one else will attempt. She tells him her father is dead; he tells her his own father does not speak to him.
Under pressure from Courtney, Sam enrolls at North Shore Community College for accounting. She also takes an Earth Science elective taught by Professor Martin Green, known as Doc Martin. Doc Martin recruits her after she wanders into his lecture. Sam splits time between Justin's house in Gloucester, which he shares with his great-grandmother Ann, and her family apartment. Ann gives Sam a mineral collection and a geology book. Sam excels in geology, earning a 98 on her midterm, while her accounting grade dips. Doc Martin tells her she has a talent for the subject.
When Justin's mother, Beth, returns from a church mission and pressures the couple toward marriage, Sam refuses. When Justin pushes her to climb instead of study, she drives home and methodically corrects every mistake on her accounting quiz, finishing the semester with an A minus in statistics and As in her other classes.
Ann breaks her hip and is hospitalized. She urges Sam to become a scientist. Outside the hospital, Sam tells Justin about her anger at Mitchell and her desire to study geology, admitting she has already applied to transfer to UMass Amherst. When Sam confesses to Courtney, Courtney is hurt but supportive, saying she herself would stay, "but you're not me" (306).
All summer, Sam works while attempting the tipped boulder with Justin on Sundays. At a farewell gathering, she tries the boulder dozens of times. In the evening light, she finally solves it, shifting her weight beneath the overhang and pulling herself to the top. She names the problem Dreamer, for her father. Standing in the parking lot, Sam says she wishes she could start from the beginning and figure it out all over again. As they walk toward the highway, she tries to memorize the leaves, the words, the light, and the air before she goes.