Having suffered from anxiety attacks since she was eight years old, creating monsters has become a sort of therapy for Marion Jones. When she sculpts and paints, her fingers don't tingle, the heaviness lifts from her chest, and she can actually breathe. So instead of grabbing pizza and milkshakes after school like most other kids in South Haven, Marion holes up in her room, working away on her monsters. And it's paid off, because she's just finished her first full-face mask: a gruesome sea monster she's named Winston.

However, Halloween arrives with a Super Blue Blood Moon, and its powers somehow manage to bring Winston to life. To make matters worse, Marion's crush, Tyler Dash, becomes the object of Winston's possession as soon as he tries on the mask, turning him into a red-flannel-and Converse-high-top-wearing creature from Connecticut. Marion has no choice: She has to follow Tyler to the Halloween dance to try and stop him. Will she be able to figure out what's happened to Tyler and stop the monster from wreaking further havoc on the school? Or has she made a real-life monster that's here to stay?