Marty Onster's parents sign him up for soccer hoping it will bring out the monster in him, but instead, Marty learns the importance of being a team player.
No one is sure if Marty Onster is really a monster, but when Bart Ully starts to make trouble during library time, which happens to be Marty's favorite time of the day, Marty's monster mania surfaces.
Marty Onster lets the monster in him loose when bully Bart Ully endangers his friend Sally by forcing her to sled over a jump at the bottom of Breakneck Hill.
Marty Onster's father decides to sign him up for Scary Scouts to see if he can develop the boy's monster skills, and while Marty is not pleased to be at camp learning to howl and screech, the monster in him comes out when he learns bully Bart Ully is at a camp across the lake.