Inspired by "The Babysitter Twins" books, Lisa decides to become a babysitter. However, because of her age, no one takes her seriously. One day, Ned mentions that Maude has been taken hostage in Lebanon and he must leave to get her released and he agrees to let Lisa babysit Rod and Todd. After a relatively uneventful night, Ned puts out the good word for Lisa, who experiences a business boom.
Meanwhile, the Springfield Squidport reopens and throws a gala. Marge and Homer decide to go and they leave Lisa to babysit Bart and Maggie. Bart is offended by the fact that his younger sister is going to be babysitting him. To complicate things further, Bart torments her by, among other things, ordering an oversized sub, hiring Krusty for a bachelor party, dialing 9-1-1 for a "sisterectomy", and having Maggie eat coffee ice cream for dinner. Eventually, Lisa angrily attempts to jump at Bart, but misses, causing him to fall backwards down the stairs, dislocating his arm, and forming a large bump on his head. Bart then realizes that if Lisa does not take him to a hospital she will not be able to babysit anymore. To make his condition worse, Bart locks himself in his room and hits his head on the wall "to make the lump bigger", then falls unconscious.
Lisa tries to call Dr. Hibbert, but thinks better of it when she realizes that her reputation as a good babysitter will be ruined. She then tries to take an unconscious Bart and hyperactive Maggie to Dr. Nick Riviera's clinic. There is a long line in the waiting room, and Lisa fails to get an appointment. She finally decides to take Bart and Maggie on foot instead of in an ambulance to the hospital. After a while, Bart falls down a cliff and into a muddy river right in front of the crowded Squidport. Everyone assumes that Lisa murdered Bart and that she is on drugs, and is about to drown Maggie. It is then thought to be a dream and Bart gets treatment and apologizes to Lisa for jerking her up and ruining her babysitting business. She forgives him, but is still distraught that her reputation as a babysitter is now ruined, though she receives some requests from Ned and Dr. Hibbert (who is now aware of the incident) later on due to lack of time to spend with their children which Lisa is delighted.