With Halloween approaching, Junie B. needs to find a costume that will scare off monsters that she believes will be out on the holiday.
At Halloween, Junie B. tries to scare people because she thinks some of them are monsters. She starts out having a secret fear of Halloween although the secret gets out quickly and May teases her. Despite her fear of Halloween and fear of clowns, she decides she wants to be Squirty, an infamous clown from the circus who scared her and her family. Junie B's mom knows she hated Squirty and wonders why she would ever want to dress as him, but Junie B. says she wants the costume to scare away all the Halloween monsters.
At the costume shop, she is unable to find the exact costume apparel Squirty had, so her mom tells her she'll just have to make up a clown to dress as. Her new clown name is Screamy and he scares everybody, not just the monsters. However while she trick-or-treats, nobody is scared of her and her costume and this annoys her. Also, everyone likes Ollie's cow costume and they're too obsessed with his cuteness to care about Junie B and give her decent candy.
After a trick-or-treat with nobody getting scared, Junie B. is annoyed and goes to sleep angrily, despite the fact that she faced her fear of Halloween. Her dad walks into her bedroom at night and freaks out because he didn't realize that Junie B. still had her clown makeup on. Her mom tells her to wash it off because she really could have scared him to death, but he laughs it off and Junie B. is happy that her clown appearance actually scared somebody.