In 1957, the lives of second-graders Bryce Loski (Callan McAuliffe) and Julianna "Juli" Baker (Madeline Carroll) intertwine in a way that will change them forever. For Juli, it is love at first sight; however, Bryce is hesitant and girl-phobic, leading him to avoid Juli at every turn. Over the next six years, their paths continuously cross, living across the street and attending the same school, which makes it difficult for Bryce to shake her off completely.
As sixth grade rolls around, Bryce concocts a plan to distance himself even further by asking out the girl Juli despises the most, Sherry Stalls (Ashley Taylor). Initially, this tactic provides temporary relief until Bryce's best friend, Garrett (Israel Broussard), spills the beans to Sherry about his intentions. Juli, hurt but unwilling to give up, returns to her Bryce-obsessed ways. In her mind, she has always believed that Bryce harbored feelings for her but was merely shy; she cherishes the memory of the day he "held her hand" in second grade, mistakenly thinking their first kiss was imminent.
Fast forward to 1963, seventh grade brings new complexities. Bryce's grandfather, Chet Duncan (John Mahoney), moves in and sees something unique in Juli, hoping to nudge Bryce's heart toward hers. They frequently discuss Juli, who harbors a deep affection for an old sycamore tree that the community struggles to understand. When town officials decide to cut it down, Juli takes to the tree, refusing to come down until her father, Richard (Aidan Quinn), persuades her otherwise. Though devastated, Juli finds solace in a painting of the tree gifted by her father, which prompts her to share her love for it publicly.
Determined to create beauty in her surroundings, Juli begins raising chickens and sells their eggs to neighbors, including the Loskis, only to be heartbroken when she discovers Bryce has been discarding them due to his father's fears. Despite the state of her yard, Chet steps in to lend a hand, forging a bond reminiscent of his late wife. As Chet encourages Juli to embrace her curiosity and creativity, Bryce gradually becomes aware of his feelings for her.
While Juli’s connection with Chet grows deeper, she begins to question her own affections towards Bryce. A visit to her uncle Daniel on his birthday results in a challenging yet enriching experience, stirring up questions about her feelings. However, when she overhears Bryce making fun of her and Daniel with Garrett, her interest in him vanishes. As Juli asserts that she no longer wants to communicate with Bryce, their families arrange a dinner that turns tense due to Bryce's father's rude comments.
As an annual charity event—the school basket boy auction—approaches, Bryce unwittingly finds himself in an uncomfortable position. Juli mistakenly seems to be bidding for him, but instead bids on another boy, leaving Bryce feeling envious as he watches her enjoy herself. An impulsive attempt to kiss Juli only pushes her further away, prompting her to flee home in tears on her bike while their friendship unravels in the background.
In the wake of Juli’s distress, Bryce’s attempts to apologize escalate from ringing the doorbell to climbing through her window, leading to a moment of revelation. When Juli discovers Bryce digging a hole in her yard, she learns that he intends to plant a baby sycamore tree as a gesture of redemption. Moved by his intentions, Juli joins him to plant the tree together, symbolizing not only growth in their relationship but the tender connection they’ve always shared.
