Flora, an eleven-year-old fourth grader, narrates her life with her younger brother Julian, her adoptive mother Emily (whom Flora calls "Mom" aloud but privately thinks of as "Person," because the word has been attached to too many people), and Dad (Jon). Flora and Julian spent years cycling through foster homes before Emily adopted them. They call themselves "Onlys," meaning they are the only constant each has ever had. Throughout the novel, Flora intersperses fantastical "theories" about where she and Julian came from: the bottom of the ocean, dogs, television, the horizon. These origin stories reflect the siblings' shared belief that they were never born, since they have no memory of being babies, no birth mother, and no first family.
Flora struggles to express herself verbally, a side effect of childhood trauma. Words either get stuck inside her or tumble out in the wrong order. Julian speaks fluently but masks his fear behind a forced smile and hoards food in his closet, terrified he will go hungry again.
At school, Flora watches the class mouse give birth. Her teacher, Ms. K, explains that human mothers love their babies forever. Flora blurts out a question about mice "like me" that were never born from a mother, but she cannot articulate what she means. That night, Flora overhears Person and Dad whispering about news that will "destroy" the children. Julian shows Flora postcards he intercepted from Gloria, their last foster mother, and suspects Person has been hiding others.
Ms. K apologizes and invites Person in to discuss Flora's question. Flora explains that she and Julian believe they were never born. That weekend, Person insists all people are born, but Flora counters that Person cannot remember being born either. Flora also reveals she and Julian have no Lifebooks, scrapbooks of a foster child's history, and no first family.
At dinner, Person announces her pregnancy. Julian bursts into tears, begging to stay. Person reassures them they are hers forever, but Flora recognizes Julian's recovery smile as fake. That night, Flora worries Person will love the baby more because it will come from inside her.
Flora's schoolwork deteriorates. Person takes them to their therapist, Dr. Fredrick, who learns the children believe they were never born. Julian erupts about the stolen postcards. Dr. Fredrick tells Person she must return them and share everything she knows about the children's past, since a baby is coming whose entire history will be known from the start.
Person shows the children her sparse paperwork. Before Gloria's house, they lived with a woman who intended to adopt them but did not. Neither the woman nor the agency can locate the children's earlier files. Flora and Julian describe their earliest shared memory: a white house where Flora turned off a light switch in a line of children, and Julian sat at a tiny table, hungry. They ask Person to help them find it.
Tensions escalate with Elena, Dad's twelve-year-old daughter from a previous marriage, who insists the new baby will be more hers because Dad is her biological father. Flora secretly enlists Elena to clean out Julian's food stash. When Julian discovers the food is gone, he confronts Elena at recess. Elena laughs at him, and Flora, overcome with guilt and rage, punches Elena in the face. Flora's words shut down for two days. A family meeting with Elena's mother, Meredith, collapses; Meredith calls Flora and Julian "dysfunction" and tells Dad that adopting them "broke our lives."
Dad proposes that Person take Flora and Julian to Maryland to visit their former foster homes while he and Elena have time together. At a therapy session, Flora admits she wants to see younger pictures of herself, and Julian says he wants to know about "before." Person commits to the trip. On the last day of school, Ms. K gives Flora a card revealing that a mouse the class named Castillo, Flora's pre-adoption last name, will be the fifth-grade class pet.
Person drives the children to Maryland and proposes a game: They will look for evidence supporting the children's origin theories versus her belief that they were born. At Gloria's house, Gloria greets them warmly and is amazed Flora now speaks. She finds two photos but has no information about where the children lived before her. When Gloria lets her dogs inside, they walk directly to Flora, who remembers all their names. Person promises Flora a dog.
They next visit the woman who intended to adopt them. She admits she lost or donated their Lifebooks and reveals she wanted to adopt Julian alone but not Flora, whom she considered unable to function because Flora barely spoke. Person declares "ELEPHANT," the family's code word for leaving an unbearable situation, and marches the children out.
After a recovery day, Person drives them to a brick house with blue shutters. Two women, Margie and Vanessa, rush out and embrace the children. This former couple cared for Flora and Julian from about ages three to eight, teaching them to swim and enrolling Flora in speech therapy. They lost custody when a judge ruled their two-bedroom house inappropriate once Julian could no longer share a room with Flora. They believe the ruling was discriminatory, based on their being a same-sex couple. Margie hands Person two Lifebooks she made for the children. Person weeps, since the previous woman had claimed these were lost.
Over dinner, Person defines family as "the small group of people who are bonded to me, who I'm choosing to love forever" (253). Flora calls Elena and cannot say "sorry" but tells her "divorce stinks," acknowledging their shared pain. On a walk, Flora confesses she does not call Emily "Mom" inside her head but "my person." Person replies, "I'm your person. One hundred percent" (287).
Person arranges brunch with Kelly, the children's former caseworker, and confronts her about how the system failed. Julian, overwhelmed by Person's anger, throws a water glass and runs out. Kelly reassures Flora that worrying about a new sibling is normal.
Dad and Elena join them, and the family drives to a tan house on a corner. Inside, everything is white. Flora and Julian confirm this is the white house from their earliest memories. The foster mother, Jeannie, runs a rigid, silent household where children eat tiny portions without speaking and babies stare blankly, never looked in the eyes. Jeannie's records show toddler Julian arriving holding a red rubber ball, and three-year-old Flora in a pink dress, hugging a doll and smiling at the camera. Someone dressed them and taught Flora to look into people's eyes. The records state only that the children were dropped off at 3 a.m. by a government official in a case of neglect. The state never found the previous caretakers.
Person tells the children she believes they were born and that they had a birth mother who loved them but could not care for them. She points to the pink dress, the doll, the ball, and their ability to love each other as evidence. Flora accepts this, deciding the pink dress and her brother's enduring love are proof enough. Julian rests his head on her shoulder: "We have until forever, I guess" (306). The family of six, Flora counting the unborn baby, leaves the white house for the last time. In her final theory, Flora acknowledges she and Julian come from hidden corners where things get stuck and lost, but concludes, "Person, and people, love us anyway" (309).