The novel opens in August 2004 with two 20-year-old Black women, Milly and Val, rowing a rented kayak to a secluded swimming spot off the coast of Cassis, France. They are study abroad classmates, still getting to know each other. When they agree to dive in on a count of three, Val leaps headfirst while Milly hesitates on the rocks. Val swims back and tells Milly that next time, she must jump with her, establishing the dynamic that defines their friendship: trust, daring, and the question of who will follow whom.
Part I shifts to North Carolina in 2018, where Val, 36 weeks pregnant, drives to her parents' house to say goodbye before moving to New York with her husband, Nic, a Chilean-born anthropologist. Her parents refuse to open the door. Humiliated, Val calls Milly, who comforts her from Brooklyn. Both are in their third trimesters and plan to raise their daughters near each other. Before the move, Nic revealed he had signed a full-time teaching contract without telling Val, breaking their agreement to share childcare. Val is furious but forgives him on the drive north.
Val and Nic settle into a small apartment in Park Slope, Brooklyn, near Milly's brownstone. Milly's husband, Harris Buchanan, comes from a wealthy family that owns a chain of East Coast restaurants. Milly has built a career as a lifestyle influencer and takes Val on a week of adventures before the babies arrive, paying for everything. They visit Queens, where Milly was raised by her mother, Gladys, a Colombian immigrant who worked multiple jobs and died of lung cancer at 43, during Milly's sophomore year.
Val goes into labor while home alone. She calls Milly, who arrives, makes Gladys's soup, and tracks Nic down through the university. Val names her daughter Helene, after the woman who brought them together in France.
An extended flashback to August 2004 reveals how the women became close. On the Seine, Milly, grieving Gladys's death, approaches Val, the only other Black girl in their program, and invites her to drive south. In Hyères, Val's father calls to berate her, and she stumbles off a cliff into the sea. Helene, a 38-year-old American at the hotel with her girlfriend, Elodie, a French medical student, rescues Val and takes them camping in the Gorges du Verdon, a canyon in Provence. At a campfire, Val reveals her abusive home life: her mother's violence, a car crash, and her flight to New York. At a forbidden swimming hole called the Styx, Milly tells Val to ignore her fear. They clasp hands and leap.
Back in 2018, Milly's daughter Astrid is born after a grueling 30-hour labor, but Milly feels no instant recognition and keeps Val away for days. She reconnects with Jerome, a high school classmate who is now a librarian and poet, and reveals that she and Harris have an open marriage, a detail she has kept from Val. In a rose garden in Prospect Park, Jerome reads her a poem, and they kiss. Milly hires Cleo, a 29-year-old Dominican woman from Flatbush, as Astrid's nanny.
Val, meanwhile, struggles with new motherhood. Her daughter, nicknamed Leeny, refuses to breastfeed, so Val pumps every two hours. Nic has returned to teaching, leaving her alone from morning to night. At a dinner hosted by the Buchanans, Val lashes out at Milly, saying it is easy to be gracious with so much help and invoking Milly's luck in having Gladys as a mother.
A second flashback returns to December 2006, when Milly and Val visit Helene and Elodie in Paris for Christmas. Helene is declining, taking sedatives, barely leaving bed. At a party, Milly vents about Val's fragility to Elodie on a balcony, not realizing Val is listening through a second door. At Christmas dinner, Milly confronts Val publicly. Elodie mediates, telling Milly that everyone needs someone who believes they are better than they are.
In the present, Milly proposes a nanny share in which Cleo cares for both babies, covering the extra cost so Val can finish her book. Jerome grows dissatisfied with being kept secret and ends the relationship after learning about the Buchanans' new restaurant, Milena, from a magazine. Devastated, Milly writes a raw personal letter about Gladys, a miscarriage she endured the year before Astrid, and her grief. She publishes it online, and it goes viral. A book agent named Pilar contacts her about a memoir.
Val's resentment of Nic reaches a breaking point. One night she accuses him of infidelity. Nic blocks the bathroom doorway; she pushes against him, he restrains her, and she falls, hitting her head on the bathtub. At the hospital, Val tells the nurse she fell, the same lie she told as a child. Milly arrives, and they fight. Val accuses Milly of being the loneliest person she knows; Milly accuses Val of never reckoning with the grief Helene's death left between them. Val tells Milly to leave.
A third flashback reveals Helene's death. In January 2010, Milly and Val fly to Provence for Helene's 44th birthday. They swim in an inland sea, Helene wearing only a white bathing suit. Val wakes from a nap to find Helene gone. Milly swims toward a flash of white beyond the breakers but cannot reach her, and seeing Val struggling, turns back. Rescue teams recover Helene's body. After the funeral, Elodie reveals Helene left them an inheritance split unequally: 70,000 euros to Val, 30,000 to Milly. Milly sees the imbalance as proof that Helene loved Val more. She leaves to travel and starts a blog. Milly's departure nearly ends their friendship.
In the present, Val tells Nic to move out. At the Milena restaurant opening, Milly and Val reconcile. Val apologizes for keeping Helene's money; Milly admits the worst part was that Helene gave Val proof of love Milly could never provide. Val presses a card of Saint Philomena, patron saint of lost causes, into Milly's hands. The night also brings a rupture with Cleo, who asks Milly for a $20,000 advance to help her mother. Milly refuses, and Cleo storms out, accusing both women of believing their proximity to wealth makes them better than others.
Cleo quits and moves to Florida with her girlfriend, Vivienne. Milly secures her book deal and tells Harris she wants to reveal their open marriage in the memoir; he reluctantly accepts. At Astrid's christening party, Milly is distracted and does not notice Astrid fall face-down in a kiddie pool. Val spots Astrid through a window, runs out, and performs rescue breathing. The baby recovers, and doctors find no lasting damage. After the crisis, Milly drives to Jerome and tells him she wants to stop keeping secrets. He takes her back.
The novel concludes with Milly and Val traveling to France with their daughters. Val lights candles for Helene in basilicas, telling Leeny the story of the woman she was named after. They meet Elodie, who has remarried and has a son; they share an unspoken understanding that they will not meet again. On a sunlit beach, Val discovers she is pregnant with Nic's child. Jerome texts Milly a poem: "Love is no antidote [...] Only pressure on the wound" (352). In the final scene, they carry their daughters into the sea. Val says she wants to keep loving Helene, and Milly tells her to love her. They stand in the water, their girls between them, the tide pulling gently around their bodies.