It is late spring and school is about to let out for summer vacation, but Charlie Brown is upset about everything, mostly due to feeling disliked and ostracized by his schoolmates. On the way to school, he meets Linus and tells him he is frustrated that he cannot enjoy himself like all the other kids at school. But when he notices the Little Red-Haired Girl sitting on a passing bus, Linus immediately realizes that Charlie Brown is in love. Sally does appear and is in love with Linus as usual.
During the next-to-last day of school, Charlie Brown agonizes over the Little Red-Haired Girl, trying to think of different ways to get her attention, only to have each attempt fail embarrassingly. First he writes her a love note, but he gets called on by the teacher to read a report in front of the class, and accidentally reads the note aloud to the raucous laughter of the class. Then he goes to the pencil sharpener and accidentally sharpens his ball point pen. Lunch hour is just as frustrating. He tries to get up the courage to go and talk to the girl, only to fail at the last moment. Then he panics when the Little Red-Haired Girl approaches, tying his peanut butter sandwich in a knot and pulling his lunch sack down over his face. On the way home, Lucy and Violet jeer at Charlie Brown for a ridiculous answer he gave in class. Linus defends him by revealing that he loves the Little Red-Haired Girl, but this only gives the girls another way to humiliate Charlie Brown. At home Charlie Brown finds out that his sister is wearing her graduation clothes and Sally tells Charlie Brown that she is graduating kindergarten and is going to the 1st grade.
That afternoon, Charlie Brown goes over to Lucy's psychiatry booth, but Lucy, already upset over Schroeder ignoring her after she breaks his piano and Beethoven bust, is not much help. Later, he meets Peppermint Patty and talks about the girl at school, but before he can mention her red hair, Patty jumps in and tells Chuck that she will set up a meeting with her. Patty then tells Lucy that "someone" wants to meet her at the ball park that night. Lucy agrees, thinking the "someone" is Schroeder, and Patty returns to tell Charlie Brown everything is all set. When the two meet each other at home plate, they both respond with "You! Bleah!"
The next morning, the last day of school before summer vacation, Charlie Brown plans to get up at 4:30 a.m. and meet the Little Red-Haired Girl at the bus stop. But he falls asleep on the bench, and when the bus comes, it leaves him behind. He arrives late at school and is sent to the principal's office right after yelling at the teacher when asked why he was late. When he comes back to class, the teacher calls on him to solve a math problem on the blackboard. Thinking he will finally impress the Little Red-Haired Girl, he struts to the blackboard and works hard, writing college geometry formulas, but when the teacher asks him if he knows what he is doing, he sheepishly admits he has not a clue. He is once again laughed at, and once again goes slowly back to his desk.
At noon school is finally over, and determined to finally meet the Little Red-Haired Girl, Charlie Brown beats everyone out to the school bus to meet her. But a clamoring crowd of fellow students pushes him away. The last student gets on the bus, and it pulls away again, leaving Charlie Brown behind. He shouts out his misery until he notices a sheet of paper that had gotten slipped into his hand. It reads:
I Like You, Charlie Brown. signed Little Red Haired Girl
Charlie Brown's anguish quickly turns into delight and hope as he dances up the hill toward home as the end credits roll. At the very end, he stops and asks himself, "Good grief! How will I live until September?"