In the year 1957, when second-graders Bryce Loski and Julianna “Juli” Baker first meet, Juli knows it’s love, but Bryce isn’t so sure and tries to avoid Juli. By the sixth grade, in 1961, Bryce tries to get rid of Juli by dating Sherry Stalls, whom Juli despises. However, Bryce’s best friend, Garrett, takes an interest in Sherry and eventually tells her the truth about Bryce asking her out; she doesn’t take it well.
From Juli’s perspective, Bryce returned her feelings, but was just very shy. After finding out Bryce and Sherry broke up, she thinks she could have Bryce back. But then they reconsider their decisions as time goes on.
In 1962, Bryce’s grandfather Chet Duncan moves in with the family. Chet has different views about Juli. There’s a large, old sycamore tree that Juli loves which no one else understands why. One day, it’s cut down by a group of landscapers so a house can be built there, despite Juli’s opposition. She becomes very depressed afterwards, as the tree let her see the world in a more enlightened way. Her father gives her a painting of the tree.
Chet gets to know Juli while helping her work on her lawn. Bryce begins to develop feelings for Juli, who begins to have mixed feelings about him. When Juli finds out that Bryce has been throwing away the eggs she offered his family right under her nose, out of fear of salmonella, she feels hurt and starts avoiding him.
After visiting her disabled uncle Daniel, Juli overhears Bryce supporting Garrett’s badmouthing of her, which truly causes her to stop having any interest in him. When the Bakers are invited to the Loskis’ for dinner, Juli confronts Bryce about what he said. During dinner, they sit opposite each other; she doesn’t talk to Bryce or make eye contact with him. After dinner, she apologizes for her behavior. Bryce is confused and sad that she apologizes, because it means he isn’t forgiven and she doesn’t care enough to hold a grudge.
As the 1963 basket boy auction approaches, Juli hears that Sherry is planning to bid on Bryce against Melanie. Bryce thinks that Juli intends to bid on him because he hears that she has a wad of cash. Bryce worries about what will happen if Juli tries to start a bidding war with Sherry, but she bids on Eddie Trulock out of sympathy as nobody was bidding for him.
During the basket boy lunch, Bryce and Sherry sit at a table across from Juli and Eddie. Bryce sees that she’s having a great time with her date and gets jealous. He grabs her and attempts to kiss her, publicly humiliating her. He chases her after she dodges his kiss. Juli gets on her bike and cycles home. Garrett yells at Bryce, and they end their friendship after arguing that Bryce had “flipped”.
Bryce tries to talk to Juli, but she refused to see Bryce and wants to be left alone. Two days later, Bryce plants a sapling sycamore tree in Juli’s front yard to show her how he feels. When Juli sees the sycamore, she goes out to help him. Upon seeing him do that, she realizes that they have never really talked after all these years. As they plant the tree, Bryce puts his hand on Juli’s, and they share loving smiles.