In April 2014, Vera Van Valkenburgh drives through the night to the Catskill Mountains, a place she has not visited in nearly two centuries. She stops at a creek and touches a pistol to her chin before a local interrupts her. Vera hides the gun and explains she is moving to the area for a forest ranger job. She has rented a cottage on the property of her family's old homestead, built around 1800, which now belongs to Cate Bennington, a graphic designer, and her husband, Brian.
Vera appears to be about 26, but she was born in 1800. She, her brother Eli, and their mother, Ma, share a condition none of them understand: They do not age, cannot be killed, and heal from any wound within minutes. After discovering these abilities in 1826, the three fled the valley, leaving behind Eli's wife, Lotte, and beginning almost two centuries of drifting under false names. Vera has returned to search for the source of their immortality, a natural spring she believes she can drink from to reverse her condition and finally die.
Vera begins training with her partner, Ranger Daniel Lopez, while quietly collecting water samples from mountain springs and testing each one on herself. Her body rebounds every time. She learns that a company called Fountain of Eternal Youth LLC, backed by tech millionaire Matthew Barbery, has purchased the old general store next door and surrounding land. The company's registered agent is a woman named Lydia Kirke. In Cate's house, Vera discovers an 1861 photograph of Lotte with a young woman on the steps of "Kirke's General Store" and steals it, connecting the image to Lydia Kirke.
At a Town Planning Board meeting in July, Lydia appears to present the LLC's proposal, and beside her sits Eli, introduced as her "life partner." He spots Vera but they pretend to be strangers. That night, Eli comes to Vera's cottage and reveals that Lydia is "one of us," another immortal and a scientist who has spent lifetimes studying regeneration. Lydia believes isolating the source of their condition could cure diseases and reverse their immortality. Vera agrees to help, privately motivated by the prospect of dying rather than by the humanitarian mission.
In the general store, now a lab funded by Barbery, Lydia delivers a scientific briefing on cell immortality. When Vera presses about reversibility, Lydia replies "quite possibly." Vera commits to the project, giving blood and tissue samples while testing water on herself in secret. Through the summer, she works her ranger shifts and helps Cate and Brian build a cidery in their barn. She also grows closer to Paul, an emergency medical technician (EMT) and volunteer firefighter who resembles Jacob, a ranch hand Vera loved and lost in Wyoming decades earlier.
Tensions with Lydia deepen. Vera discovers Lydia's diary from 1863 in the town archives and steals it. Its final entry records that Lotte showed Lydia a gift of dominoes and believed Eli was alive, contradicting Lydia's claim that she never knew Lotte. In a separate confrontation, Lydia asks Vera directly whether she wants to die, revealing she has deduced Vera's suicidal pattern. On the creek, when Vera warns Eli to be careful with Lydia, he erupts, accusing her of hating every woman he loves and calling Jacob's death a "blessing in disguise." Vera storms off.
During an October game night, the power goes out and Vera panics. In her cottage, Eli discovers her hidden stash of water samples. Vera breaks down and tells him about the landslide at Joshua Tree National Park that buried her under boulders for four and a half days, her body burning as it tried to heal under the crushing weight. Eli had seen her note but left to be with Lydia. Vera was rescued but psychologically shattered. Eli is devastated and holds her. That night, Paul stays over, and he and Vera sleep together for the first time.
In November, during a patrol at the Hunter Mountain Fire Tower, Lopez asks Vera to be his baby's godmother. She agrees, but moments later he slips on ice, crashes through the railing, and falls 60 feet. Vera dives after him, landing beneath him to absorb the impact. Impaled by a piece of railing, she heals, pulls it free, and tends to Lopez's compound fracture. Believing he is dying, she tells him the truth: She is 214 years old and cannot die. Lopez survives, but his concussion blurs his memory of her confession. He remembers only that she agreed to be godmother to his daughter.
At a dinner with Matthew and others, Matthew announces that Lydia has created "the world's first immortal cell." Afterward, Vera confronts Lydia in the lab. Lydia admits she gave Matthew a set of Vera's cell cultures without consent and reveals to Eli that Vera has been helping only so she can die. Matthew, eavesdropping through the lab's security system, steps out and declares both women assets who can never leave his sight. Vera shoots Lydia through the chest with her derringer, a small pistol, to prove her immortality. Eli takes the gun and shoots Matthew in the head, his first kill in two centuries.
The three douse the lab in gasoline and set it ablaze. As they watch, Vera forces the truth about the diary into the open. Lydia admits she lied about not knowing Lotte: Lotte spent her life waiting for Eli, who secretly sent her gifts for years, and Lydia hid this to protect him. Lydia then runs back inside to retrieve her diary, and an explosion rips her body apart in a scenario they have always dreaded, where consciousness persists as agonized fragments. Eli and Vera find her charred remains and carry her inside. She takes two days to heal.
After a staged funeral, Eli tells Vera he will buy the land from Fountain's investors, sell it to his own shell company, raze the ruins, and leave with Lydia to ride out 50 to 70 years abroad. Vera begs him to stay, but he insists he belongs with Lydia and accuses Vera of always seeing the worst in their condition. He leaves without saying he loves her. Paul comes to Vera afterward, tells her he knows the fire was arson but will never tell anyone, and confesses he is falling in love with her. Vera decides to tell him her truth.
Nearly a year later, in September 2015, Vera is content in a way she has not known in centuries. The cidery is thriving, Lopez has been promoted to lieutenant, and Vera has bought the cottage from Cate and Brian. She and Paul are together; he knows everything, though they both understand she will eventually leave and he will grow old. At the cidery bar, Vera spots an unfamiliar woman who turns out to be Ma, now calling herself "Ani." Ma reveals she has been returning to the valley since 1921, searching for her children, and has spent decades using her indestructibility for humanitarian work in dangerous places. When Vera tells her about Lydia's research, Ma asks, "But at what cost?"
Ma climbs the scraggly apple tree in Vera's yard, the same one Vera climbed as a child after their father's burial, and plucks its single fruit. Paul's escaped buck, a deer he shot but could not kill the previous autumn, approaches Ma without fear. She embraces the animal, feeds it the apple, and tells Vera there are other immortals. Then she delivers the novel's final revelation: "Vera, it's not a fountain."