Lots of little brothers can be pests and troublemakers--but Amy's brother Arnie is a MONSTER ...

In this R. L. Stine family-friendly scare fest, Amy and her friend, Lissa, don't know what to do about Arnie's bullying, bad-boy behavior, and mean jokes and pranks. The little monster is ruining their lives!

The girls decide it's payback time. Total humiliation for Arnie. They find a recipe for blue slime on a YouTube channel and mix a big bucket to pour over Arnie at his birthday party.

To their horror, the girls instantly discover that SLIME DOESN'T PAY! Before their eyes, Arnie's whole body starts to change. The slime turns him into a real monster.

Now Amy and Lissa have two frightening dilemmas: Can they save their town from the raging Arnie Monster? And is there any way to turn the monster back into Arnie?