Contains spoilers for this episode and earlier episodes.
Gravity Falls holds Summerween, a summer version of Halloween complete with carved watermelons and trick-or-treating. Mabel prepares matching costumes for herself and Dipper and plans to spend the night collecting candy with Candy and Grenda. Dipper changes his mind after Wendy invites him to a teenage party. Embarrassed by the idea of being seen in costume, he pretends to be sick and dismisses an unwelcome visitor. The visitor is the Summerween Trickster, who threatens to eat the children unless they collect five hundred pieces of candy before the last jack-o'-melon goes dark. Dipper joins the hunt, still hoping to reach Wendy's party afterward.
The group nearly meets the quota, but Dipper hides their candy to avoid being caught trick-or-treating by Wendy and loses it. Mabel discovers his lie, and the Trickster returns when their time runs out. Soos helps them flee, but the creature pursues them into the Summerween store. Dipper admits his fear of looking childish, while Mabel explains that growing older gives them fewer chances to enjoy the holiday together. The monster swallows Soos and reveals that it consists of discarded, unwanted candy. Soos eats his way out and praises its taste, giving it the appreciation it wanted before it dies. The friends return home, where Dipper willingly shares candy with Mabel rather than leave for the party.