The novel begins where It Ends with Us left off, after the novel’s protagonists Lily Kincaid and Atlas Corrigan run into each other on the street, and Lily tells Atlas she and Ryle are now divorced. Told in chapters that alternate between Atlas’s and Lily’s perspectives, the novel begins with Atlas nervously hoping Lily will text him soon like she said she would. This hope keeps him uplifted as he handles the reality that someone broke into his restaurant, Bib’s, and vandalized it with a misspelled profanity. Meanwhile, Lily is incredibly busy at Lily Bloom’s, her flower shop, and is confused about what to do about Atlas, knowing a romance with Atlas will infuriate her jealous and abusive ex-husband, Ryle, with whom she shares jointed custody of their infant daughter Emerson.   

Atlas visits the flower shop wanting to talk to Lily and ask her if they can speak on the phone that evening. Ryle comes in unexpectedly, and a panicked Lily hides Atlas in the office supply closet. Lily and Atlas do talk on FaceTime that evening, catching up on each other’s lives, flirting, and testing the waters of their relationship. Hearing that Ryle responded in rage after reading Lily’s teenage journals about Atlas, Atlas asks Lily to read to him from the journals. 

On their first date, Lily gifts Atlas one of her journals. Exhausted after staying up all night with a sick Emerson, Lily falls asleep during the ride to the restaurant. Atlas lets Lily sleep, content to read her journal, reliving their first kiss. Inspired, Atlas writes Lily a letter about his life before her. Atlas recounts how his mother rejected and abused him as a teenager, choosing herself and her abusive husband Tim over Atlas. He says that Lily changed his life and taught him what love was. When Lily wakes up, she reads Atlas’s letter and is moved by it. They want to kiss goodnight but decide to continue taking it slow. 

After his date with Lily, Atlas encounters his mother, Sutton, who is at his restaurant to ask Atlas if he knows where his twelve-year-old brother Josh is. Atlas tells Sutton he didn’t know Josh existed until their conversation. Atlas realizes that Josh is likely the vandal targeting his restaurants. As Atlas deals with Sutton, Lily returns from their date to an angry Ryle, who has just watched Finding Nemo for the first time. Ryle is furious that his daughter’s middle name, Dory, is associated with Atlas, as he is aware that Lily and Atlas often talked about Dory as teenagers. Ryle’s rage-fueled confrontation shakes Lily, and after getting away from Atlas, she is comforted when Marshall, Ryle’s best friend and brother-in-law, tells her she was right to leave Ryle. After their respective rough nights, Atlas drives to Lily’s house to give her a hug. Afterward, Atlas stakes out Bib’s and reads Lily’s journal entry about the first time he and Lily made love, after which Lily’s father almost beat him to death. Atlas catches Josh attempting to vandalize his restaurant. He convinces Josh to come stay with him. Atlas doesn’t want Josh to return to live with Sutton, who he feels is still abusive and only cares about herself. The following day, Sutton demands that Atlas bring Josh to her, and when he does, Atlas sees Sutton strike Josh. He immediately gets Josh back in the car and then takes him to the hospital, vowing to fight for custody.  

Lily invites Atlas to be her date for her coworker Lucy’s wedding. While Lily is getting ready, Atlas sees Lily’s collarbone, the spot where Ryle once bit Lily, and notices the heart tattoo that Lily got as a teenager. Atlas feels upset upon realizing Ryle bit Lily’s collarbone out of jealousy. Lily realizes Atlas’s anger is protective, and she isn’t afraid of him. Atlas and Lily kiss. After making a brief appearance at Lucy’s wedding, they return to Lily’s and make love for the first time since they were teenagers. Afterward, they talk about Atlas fighting for custody of Josh and about the aftermath of Lily’s father’s violent outburst the last time they made love. Atlas returns home to Josh.  

The next morning, Lily is woken to the sound of Ryle pounding her door. Ryle notices evidence of the previous evening and quickly figures out that Lily was with Atlas. He flies into a rage, pinning her to the wall by her throat. Lily, afraid that he’s going to come for Emerson, tells her mother, who watched Emerson the previous night, to take her to a park. Unsure where to go after she picks Emerson up, Lily takes Emerson to Atlas’s and hides out. Josh is working on a homework assignment about his family tree that he struggles to finish because he doesn’t know his family. Atlas thinks if he and Lily were together, he’d raise Emerson as his own daughter. 

Lily wakes up the next morning to a series of angry, threatening texts from Ryle, who also comes to the flower shop to try to make nice after his drunken, menacing texts. Lily tells Ryle she’s sent the messages to her lawyer. Ryle leaves in a rage, and Lily warns Atlas that Ryle may be coming for him, and before Atlas hangs up, Ryle is there. Ryle punches Atlas, blaming him for his problems with Lily. Atlas nonconfrontationally points out to Ryle that his marriage ended because of his temper. Atlas tries to convince Ryle that they need to be a team, since Emerson is impacted by all of their behavior. Ryle leaves, somewhat less angry. Atlas tells Lily about the conversation, and she is pleased that he diffused the situation with words and not violence. 

Atlas and Lily are having a peaceful morning together when Sutton arrives at Atlas’s, demanding Josh back. Josh returns from Theo’s and is afraid of his mother. He insists he wants to live with his father, Tim. Sutton leaves and Lily connects with Josh while Atlas calms down. Atlas takes Josh to see his father in Vermont. Josh realizes he wants to stay with Atlas and says his family tree can be a seedling with two branches—one for him and one for Atlas. Later, Allysa, Marshall, and Lily confront Ryle, telling him that he needs to go to anger management and that he’ll only have supervised visits with Emerson and Rylee until further notice. Ryle is upset but agrees to get help. Meanwhile, Atlas tells Sutton that he wants custody of Josh but encourages her to meet them once a week for dinner, so she can still have a relationship with both her sons. Six months after they began dating, Atlas and Lily move in together. A few years later, they get married. In his vows, Atlas says that no matter what happens, he’s grateful to have gotten to love Lily and even to have her break his heart. He is hopeful for the future.