There is a new student in Lisa's class, Allison Taylor, who seems pretty smart. However, Lisa learns that she is younger and has been skipped ahead a grade. Furthermore, they both play sax and are vying for first chair in the school band. After they both try way too hard, Lisa faints, and Allison gets the position.
Meanwhile, Homer comes across an overturned truck of sugar and steals all the contents. He tries to sell it door-to-door, but has no luck. Marge insists that he get rid of it, but he refuses. After guarding it day and night, a rainstorm melts it all away.
Lisa tries to be friends with Allison, but her jealousy gets the better of her. Bart persuades her to sabotage Allison's entry into the school diorama contest by swapping it with another. Lisa becomes overridden with guilt, and uncovers the real one, but Allison still doesn't win - Ralph Wiggum does! Lisa apologizes and they both become friends.