Lucy and Linus go out one evening to go find a pumpkin for Halloween. At the pumpkin patch, Lucy picks the biggest one, almost as big as Linus, and has him carry it back home. After putting the pumpkin down in some newspapers, Lucy proceeds to carve it. As she scoops out the insides, Linus protests: "You didn't tell me you were gonna kill it!"
Charlie Brown is raking leaves in his yard, while his dog Snoopy helps by blowing the last leaf into the pile. Linus, who is eating an all-day sucker, jumps on the pile, getting leaves stuck all over the sucker and his face. Lucy invites Charlie Brown to kick a football, but he refuses, knowing that she'll just pull it away and watch as he falls flat on his back. Lucy produces a signed document testifying that she won't pull it away, so Charlie Brown takes a running start. She pulls it away anyway, explaining that the document wasn't notarized.
Linus is writing a letter to the Great Pumpkin, whom he believes rises up from the pumpkin patch on Halloween night to bring presents to all good girls and boys. Everyone who passes by thinks he's wasting his time, except for Sally, who has a crush on him. Undeterred, he finishes the letter and mails it. Charlie Brown comes in with a letter of his own, an invitation to Violet's Halloween party (which Lucy claims was sent to him by mistake). Meanwhile, Linus takes his place at the pumpkin patch to await the Great Pumpkin's arrival.
On Halloween night, the gang is getting ready to go trick-or-treat. It's Sally's first time, and she's worried that it might not be legal. Lucy is dressed as a witch, claiming that it contrasts with her real personality. Charlie Brown goes as a ghost, but he had trouble with the scissors and cut out too many holes. Even Snoopy is in costume, dressed as a World War I flying ace. The kids go trick-or-treating, passing by the pumpkin patch to mock Linus. They leave, but Sally decides to stay with Linus. Linus assures Sally that the Great Pumpkin will show up, because he picked the most sincere pumpkin patch he could find.
The kids go door to door asking for candy, with Lucy asking for extra candy for her brother, while Charlie Brown somehow is only given rocks. Meanwhile, Snoopy returns home to act out his WWI fantasies. Sitting atop his Sopwith Camel (his doghouse), he takes to the air and engages the Red Baron in battle. He gets shot down and lands behind enemy lines. Snoopy sneaks around the French countryside, eventually finding refuge in a farmhouse (actually Violet's home).
After trick-or-treating, the kids all head to Violet's for the party, again passing by the pumpkin patch to make fun of Linus. Sally defends him, but after the kids leave, she demands to know when the Great Pumpkin will arrive. At the party, Charlie Brown is asked to model; he accepts, but then it turns out they just needed the back of his head to draw on it as practice for a jack o'lantern. Lucy plays bobbing for apples, but finds Snoopy hiding inside the tub and is disgusted. Snoopy leans by Schroder's piano as he plays tunes from WWI, then heads back out into the night.
Back at the pumpkin patch, Linus is telling Sally that the Great Pumpkin will show up soon. Then they hear rustling. A figure rises from the middle of the patch, and Linus is so overcome with emotion he faints. Unfortunately, the figure turns out to be Snoopy, and when Linus comes to, Sally gives him an angry tirade about how he made her miss Halloween, threatening to sue him for restitution. She leaves with the others, as Linus begs them to stay and wait for the Great Pumpkin, but even he is starting to have doubts.
Linus stays at the pumpkin patch all night, until Lucy gets up in the middle of the night to take him back home. The next day, Charlie Brown and Linus reflect on the night's events. Charlie Brown assures Linus that he's done his share of stupid things too. This upsets Linus, who vows that the Great Pumpkin will appear next year as the credits roll.