Caitlin, a sixteen-year-old in the suburban California town of Los Cerros, learns from her parents that her best friend, Ingrid, has died. She hums a single note to drown out their words, then bolts from the house and rides a bus to an unfamiliar neighborhood, where she sits on a bench compulsively picking at its wood. Her father retrieves her. Over the following days, Caitlin can barely function. Her parents drive her up the coast, and during a roadside lunch, reveal that Ingrid had clinical depression and had been on medication since she was nine, facts Caitlin never knew. She and her mother spend the rest of the summer drifting through Northern California, talking only about what is directly in front of them, never about Ingrid.
When September comes, Caitlin has begun sleeping in her car each night, unable to calm herself indoors. On the first day of junior year at Vista High, she walks to school the long way and is accompanied silently by Taylor Riley, a popular, sun-bleached skateboarder who says he is sorry about Ingrid. She retreats toward her advanced photography class, hoping her teacher, Ms. Delani, will offer comfort, but Ms. Delani ignores her entirely, refusing to make eye contact. Ingrid's three photographs hang prominently on the classroom wall, while Caitlin's single displayed photo is tucked behind a heater. In English class, a new student named Dylan Schuster arrives, tall and skinny with wild dark hair, smudged eyeliner, and safety-pinned clothes. Dylan claims the locker next to Caitlin's in the remote science building.
Caitlin eats lunch alone and refuses her mother's suggestion of therapy. Her father, hoping to give her an outlet, surprises her with a pile of sanded wood planks and a saw. While searching under her bed one weekend, Caitlin discovers Ingrid's journal, its blue cover marked with a Wite-Out bird. The first page holds a self-portrait Ingrid drew titled "Me on a Sunday Morning." Terrified and desperate in equal measure, Caitlin begins carrying the journal everywhere, resolving to read only a little at a time so there will always be something new of Ingrid left to discover.
Caitlin decides to join Dylan for lunch at a noodle place. Their friendship grows quickly: Dylan, who is queer, introduces Caitlin to her girlfriend, Maddy, and her circle of friends in San Francisco. They visit the abandoned Parkside Theater, a run-down building scheduled for demolition that Caitlin and Ingrid used to frequent. Caitlin feels guilty bringing someone new there but grateful for the companionship. At Dylan's house, Caitlin notices a snapshot of a little boy that Dylan quickly deflects attention from. Maddy privately tells Caitlin that Dylan also lost someone but does not want to discuss it.
As Caitlin reads more journal entries, the weight of guilt becomes crushing. One entry, addressed to Caitlin, reveals that Ingrid desperately wanted to tell her about the pills, the doctors, and her fears of losing her mind, but could not bear to lose the one person who saw her as normal. Caitlin catalogs every failure: not reporting the self-harm, leaving when Ingrid went quiet, looking away from the cuts on Ingrid's body. She concludes she could have saved Ingrid.
Overwhelmed by the responsibility of friendship, Caitlin pushes Dylan away, standing her up at lunch and lying about it afterward. Dylan stops speaking to her. For weeks Caitlin hides in an unused bathroom stall, sketching plans for a treehouse on binder paper. Meanwhile, her connection with Taylor deepens. When he comes over to work on a math project, Caitlin impulsively kisses him. Taylor kisses her back but stops, saying it felt weird. He returns later to reassure her that he does like her.
Caitlin begins building the treehouse, bolting a ladder to the oak tree in her backyard and hammering by flashlight. On Ingrid's birthday, she meets Jayson Michaels, the track star Ingrid loved, for coffee and tells him about Ingrid's feelings. Jayson reveals he had been thinking about Ingrid constantly, but she died before anything could happen between them. They break into the theater together and explore it.
Caitlin apologizes to Dylan, who tells her bluntly that she is not a replacement for Ingrid. Caitlin responds by declaring she chooses Dylan as her friend, and they reconcile. Ms. Delani also requests a meeting and confesses that she had hoped Caitlin would not take her class again because Caitlin's presence was intertwined with memories of Ingrid. She reveals that a drawer in her office is full of Ingrid's photographs of Caitlin, including a prize-winning portrait, and explains that Caitlin's displayed classroom photograph was placed in the top left corner of the wall because the eye is drawn there first, a position of honor, not dismissal. She encourages Caitlin to create her own final project and to sort through Ingrid's photographs for inspiration.
At a hardware store, Caitlin asks for rope thick enough to support her weight. Her mother sees the rope and panics, believing Caitlin is suicidal. Caitlin hugs her for the first time in months, promising she is okay, and shows both parents the treehouse.
In spring, Taylor and Caitlin become a couple. She passes her driving test. They attend Maddy's production of
Romeo and Juliet in San Francisco with Dylan. During the suicide scenes, Taylor whispers distractions until the moment passes. Afterward, Taylor and Caitlin share their first kiss as a couple.
Sitting in her treehouse one night, Caitlin reads Ingrid's most devastating entries, including one describing a self-destructive sexual encounter and several anguished short letters. The final entry, addressed to Caitlin, says there are no reasons, that the sadness is too heavy, and asks Caitlin to forget her and find a better friend. All remaining pages are blank. Dylan then reveals her own loss: Her younger brother, Danny, died of illness when he was three. She tells Caitlin three things about him: he loved chasing pigeons, he switched the B and D in the alphabet, and he had the strongest little arms.
Caitlin drives to San Francisco and copies the entire journal at Copy Cat, a copy store where Maddy works. She distributes selected entries to the people who loved Ingrid: She leaves pages for Ingrid's older brother, Davey, and his fiancée, Amanda. She gives Jayson copies of the positive entries about him. She visits Ingrid's parents, Susan and Mitch, and impulsively offers them the journal itself, but they gently decline, saying Ingrid meant for Caitlin to have it.
For her final project, Caitlin projects Ingrid's photographs onto the theater's movie screen and photographs them there, creating a twelve-image series called "Ghosts." Ms. Delani calls it stunning and hangs the series at the center of the classroom wall.
The night before the theater's demolition, Caitlin drives there one last time. She rips out Ingrid's self-portrait drawing and keeps it, changes the marquee letters to read "I MISS YOU," and places the journal on a shelf in the projection room. In the morning, she watches the building come down, imagining the journal's pages drenched and its words becoming unreadable. Jayson arrives and sits beside her.
After the crowd disperses, Caitlin stays alone. She thinks back to the night before Ingrid died: They were studying biology on Caitlin's bedroom floor, getting distracted by songs on the radio. Ingrid told Caitlin to plan their futures. Caitlin described wanting to study art out of state; Ingrid said she would go wherever Caitlin went. At some point that evening, Ingrid must have slid the journal under the bed. Back at the demolition site, Caitlin sets up her tripod, sets the timer, and steps in front of the camera. She stands still as the shutter opens: an almost seventeen-year-old with dark hair, freckles, strong arms from building, and brown eyes wide open, her expression part longing, part sorrow, part hope.