Sylvie Scruggs is finally in fourth grade, and that means she's old enough to join her town's junior ice-hockey team. Sylvie is thrilled to discover her years of skating pay off, as she quickly becomes one of the best players on the squad. But someone else is still better: Jamie Redmond, a fifth-grader who notoriously doesn't like fourth-graders. And, it turns out, she "really" doesn't like Sylvie.
Then someone starts pranking Sylvie at practice, loosening the top of her water bottle and replacing her special lotion with mayonnaise. Sylvie knows it must be Jamie, trying to psych her out and keep her from being selected as team captain. She enlists her friends Miranda, Josh, and Georgie to prove Jamie's guilt once and for all . . . but can they catch the mean girl before Sylvie has a meltdown of her own?