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'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.

Marty Onster tries to keep his inner monster in check when Bart Ully gives Sally, Marty's best friend, a showy Valentine's Day card.