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After a fortune telling machine tells Lisa she has middle children syndrome, Lisa tries to fit in, which is hard when Homer adopts Nelson.
Homer finds out that Moe has taken Bart's treehouse to do a bar for kids of barflies, Homer doesn't mind and then he sees a fortune telling machine and swipes it. The family take a turn at the machine, Marge's seems dark as she refuses to say it, Maggie is orally fixated, Bart gets underachiever and proud of it and Lisa's says Middle Child Syndrome. The family discuss how to get rid of Lisa's affliction, Homer suggests they either make another kid or get rid of one. However, neither happens and Lisa feels good about herself until Homer solves the problem by adopting Nelson.
Nelson then becomes protector of the family forcing Homer to the sidelines and him to become emasculated. In the meantime, due to Nelson protecting the family, all of SPringfield Elementary's kids are putting on weight which is because of 1) Nelson not bullying the kids for lunch money thus they can't eat and 2) They are eating the school lunches, minus Lisa as she is still vegetarian. SKinner realizes that with the weight gain, the school can't afford new desks and may have to close but Homer decides to fire Nelson and gives him to Willie who adopts him. The kids are now slimmer and poorer because of this. At the end, Lisa is upset because the plan to give her attention took attention away from her. Homer just gives her money to spend on a Malibu Stacy doll and Bart finishes off the story by saying "If you can't pay attention to your kids, just pay them".