Lisa's babysitting reputation is tested when she is assigned the task of babysitting Bart and Maggie, for which Bart is determined to make Lisa's life a living hell. It is also the 170th episode overall.
Inspired by reading a book about two babysitters, Lisa embarks on a career in babysitting. After many successful jobs, when Homer and Marge go out for the night, Lisa is left in charge of Bart. Bart becomes outraged with the proposition, so he plans to 'teach' Lisa a lesson that he is not so easy to babysit. First, he refuses to eat his dinner. Next, he watches TV without Lisa's approval. He feeds Maggie ice cream with caffeine in it, causing her to run rampant in a hyper state. Then, he makes outrageous orders by phone, including a massive sandwich and hiring Krusty, leaving Lisa to answer the door for each one. When Lisa finally orders him to go to bed, Bart goes to the kitchen and eats bread, claiming Lisa told him to go to bread.
Enraged, Lisa starts chasing him around the house, and accidentally knocks him down the stairs, causing a large lump to form on Bart's head and giving him a dislocated arm. However, despite the dislocated arm and the bump on his head, Bart continues to make baby-sitting hard for Lisa. Bart locks himself in his room, preserving his injury to show their parents. He then bangs his head against the door 'to make the lump bigger', causing him to fall unconscious.
Lisa considers calling Dr. Hibbert for help, but realizes her reputation as a babysitter will be ruined - Lisa decides she must get Bart to a hospital without anyone knowing. She finds Dr. Nick's number in the phone book, and climbs into Bart's room via a tree to recover him. She pops him in a wheelbarrow, along with Maggie, locked in a pet cage.
At the emergency clinic, Lisa can't get Bart an appointment, so she decides to take her cargo to the hospital by foot. After avoiding discovery by Chief Wiggum, Bart falls out of the barrow, sliding down a muddy hill. Lisa follows with Maggie and the barrow, only to have a searchlight turned on her. She has ended up at the Squidport opening Marge and Homer were attending, along with many Springfield residents, Dr. Hibbert included. The panicked onlookers assume Bart is dead, Lisa is on drugs and Maggie was being taken to be drowned. This is quickly debunked by Hibbert.
Bart gets treated for his injuries, apologizing to Lisa for being a jerk and ruining her babysitting business. She later gets calls from Dr. Hibbert and Ned Flanders, asking her to babysit for them despite her mishap with Bart.