On Halloween, the Peanuts gang gears up for trick-or-treating and festivities. Linus is determined to spend the night in the pumpkin patch to witness the Great Pumpkin, a figure he believes rises from the most sincere pumpkin patch to deliver toys to faithful believers. While Linus pens his yearly letter to the Great Pumpkin, his sister Lucy and the others focus on costumes, with Charlie Brown excited yet anxious about being invited to a party. The children mock Linus for his unwavering belief in the Great Pumpkin, but Linus remains steadfast, hoping to behold the mystical figure he admires.

Meanwhile, Snoopy, Charlie Brown's beagle, embarks on a fantasy adventure. Donning a World War I flying ace costume, he imagines himself in aerial combat with the infamous Red Baron. He is eventually forced down behind enemy lines, where he makes his way across the countryside, adding a playful subplot to the Halloween festivities. As the night unfolds, the children collect treats, encounter mishaps, and revel in the fun of Halloween, while Linus convinces Sally to stay with him in the pumpkin patch instead of enjoying the traditional activities. Her infatuation with Linus overshadows her initial reluctance as she dreams of seeing the Great Pumpkin by his side.