Jack "June" O'Donnell IV is an 18-year-old girl living with her mother Léa, her stepfather Toddy, and her two younger half-brothers in a farmhouse in Five Fingers, Michigan. The property sits on what June's late father called a "thin place," where the living world and something beyond overlap. Tiny white tufts called Window Whites drift in doorframes—supernatural portals to memories from the past—and two ghosts haunt the grounds: a warm, pink shimmer June and her best friend Hannah Kuiper call Feathers, and a cold, dark shape called Nameless that appears before something terrible happens. June's father, Jack O'Donnell III, died of a heart attack when she was eight, and her family has only two rules: never swim at Five Fingers Falls and never go near the Angert family, another founding family with whom the O'Donnells have feuded for generations.
At the annual carnival, June encounters Saul Angert, who left Five Fingers three years earlier after his twin sister Bekah's cancer diagnosis and eventual death. June collides with him in the House of Mirrors, and they banter easily before she realizes he already knows she is an O'Donnell. June, Saul, Hannah, and Saul's cousin Nate Baars spend the evening together, debating on the Ferris wheel whether the supposed curse between their families is real. Each family blames the other for a Depression-era dispute over cherry farmland. Saul later reveals he returned because his father, Eli Angert, has rapidly progressing Alzheimer's disease.
That night, June finds a vision in her bedroom: her younger self tucked into bed while Dad tells the story of their ancestor Jonathan "Jack" Alroy O'Donnell settling on their hill, where a cherry pit grew into a tree overnight. Dad vanishes when June chases him, and a Window White emerges from her palm, suggesting the Whites triggered the vision. June begins writing her father's stories for a creative writing class. Saul offers to tutor her, arriving at her house as "Mike" to conceal his identity. He praises her voice but says her stories lack conflict. When they step barefoot through a doorway while touching a White, they enter a memory of Dad and young June listening to a chicken's heartbeat while Nameless watches from the woods.
June and Saul realize the Whites hold memories activated by touching one and stepping barefoot through a threshold. They witness Bekah begging to stop chemotherapy while Nameless writhes nearby, and a beach day Saul calls his most treasured memory. Then comes the worst memory: the day Dad died. June watches Nameless wrap itself around her father in a forest clearing. Before dying, Dad speaks to the ghost: "You can have me, but leave June alone" and "Please show her" (159). Saul carries a sobbing June out of the frozen memory, and she tells him to leave.
Over the following weeks, June pursues the Whites alone. She watches her mother sit up night after night waiting for Dad to return from unexplained absences, then witnesses the real version of her parents' engagement: Mom confronts Dad after a three-week disappearance, holds out a plane ticket back to France, and reveals she is pregnant. June realizes the pregnancy, not a grand love story, kept her mother from leaving. Her feelings for Saul intensify, and they kiss for the first time.
Guided by Feathers, June sees memories spanning her father's life. In therapy, Dad describes a "darkness" inside the Jacks, the O'Donnell men named Jack across generations, that tempts them toward self-destruction. June sees her grandfather Jack II as a haunted man and Jack the First defiant on his deathbed. She witnesses Jack II as a child comforted by his older sister Issa, a red-haired girl who promises to protect him. June realizes Feathers is Issa O'Donnell, who has been guiding her through the memories all along.
On homecoming night, the group goes to Five Fingers Falls. June and Saul jump into the water, where June discovers the pool is filled with glowing underwater Whites. They surface to find the world changed: It is dawn, and Nameless looms on the cliff above. They escape into a cave behind the falls, where June hears her father's voice calling through the Whites. Convinced he is there, she dives back and nearly drowns. Saul pulls her to safety.
Soon after, June's mother discovers that "Mike the tutor" is Saul Angert. Mom grounds June and reveals that Abraham "Abe" Angert, Saul's great-uncle, killed June's great-aunt Issa when Issa was 17. Through more Whites, however, June sees a different story. Issa and Abe were young lovers who carved their names into O'Dang!, a massive hemlock tree on O'Donnell land. In the pivotal memory, they stand atop the waterfall, and Issa suggests jumping together to be reborn on the other side. They leap, but only Abe resurfaces; Issa has drowned. Abe carries her body to the farmhouse, where Jack the First beats him in grief-maddened rage. June understands that this violence broke the thin place and created the curse: Abe's shame transformed him after death into Nameless, the ghost haunting both families for generations. Issa's ghost warns June she will drown if she enters the water alone but that she and Saul must go in together to break the curse.
On the night June and Saul plan to meet, she finds Nameless thrashing near the chicken coop. She races to the Angert cabin and finds Eli alone, bleeding and raving that the ghosts appeared and Saul ran. Issa's ghost leads June to the falls, where she realizes Saul has already jumped. June leaps in, dives through the Whites, and surfaces in an unfamiliar ocean beneath a gray sky. She has crossed through the veil.
Issa, now fully visible, explains that the dead arrive here and are meant to wash their memories in the water, letting them dissolve. The families' refusal to release grief and hatred kept pulling memories back through cracks in the veil. Abe, unwilling to let go of his shame, slipped back as a ghost whose anguish lashed out at O'Donnells whenever tragedy struck an Angert. The blood coating June's skin represents inherited violence from Jack the First; Saul's bruises represent Angert shame. Breaking the curse requires someone to let go first.
Issa leads June to Saul. They wade into the water and wash each other clean: Saul pours water over June's bloodstained arms until the red disappears, and June massages water into his bruises until they fade. Abe emerges from the mist, releases a single White toward June, and mouths an apology to Issa but turns back, not yet ready to let go.
Two floating islands approach: one bearing June's farmhouse and one leading deeper into the afterlife. June and Saul are tempted to stay and search for the people they have lost, but they choose to live. They shout "SEE YOU" across the water and step toward the farmhouse. June passes through the door into Abe's memory: the night they found the dead hen, a week before Dad died, when he hid a leather-bound book labeled "JUNIOR" in a hollow of O'Dang! The memory crumbles, and June collapses on her real porch. She and Saul have been missing for three days.
In their absence, Mom visits Eli in an assisted living facility, pays his bills, and brings him dried cherries, signaling the feud has ended. Later, June retrieves her father's book from O'Dang! It contains no adventure stories, only a declaration that the most important thing he ever did was love her. His final words read: "I was just a moment, and you gave me a million Junes. I was just a moment, and you made me forever" (391).