After a long morning at church, the Simpsons go to the grocery store Eatie Gourmet's to take advantage of free samples in lieu of a Sunday brunch and Bart's suggestion that the family go Catholic so they can have "communion wafers and booze". At the store, Homer wants to buy a lobster, but since the larger ones are too expensive, he decides to buy a small one and fatten him up before he "eats the profits". Homer also tries to look for normal flavors of ice cream among the unusually named flavors at the "Ken & Harry's" factory plant, so he puts Lisa into the freezer to look for some in the back, which ultimately causes her to catch a cold.
Although Lisa hates the idea, Marge wants Lisa to stay home from school for the next few days to recover from her cold. Lisa is derisive about playing one of Bart's video games in order to pass the time, but soon becomes addicted to it, and consequently ignores the homework on The Wind in the Willows that she is given by Ralph. She even fakes the perpetuation of her illness so that she can continue playing the game. When Marge finally compels her to return to school, Lisa realizes she is unprepared for a test on the book, having not read it. In a panic she visits Bart, who brings her to Nelson, from whom she gets the test answers. Miss Hoover grades the tests over lunch, and Lisa is awarded the rare grade of A+++. Meanwhile, Homer has become attached to his lobster and names him Pinchy. When the time comes to cook Pinchy, Homer cannot bring himself to do it, and instead declares him a part of the family.
The family is extremely proud of Lisa's "achievement", although she is guilt-ridden at having cheated. The next day at school, Principal Skinner informs Lisa that her test grade has brought Springfield Elementary's GPA up to the state's minimum standard, and they now qualify for a basic assistance grant. Lisa admits that she cheated on the test, but Skinner and Superintendent Chalmers persuade her to keep it a secret so the school can keep the money. During the presentation, Lisa says that she understands about how desperate the school needs the basic assistance grant money, but also says that the truth is more important, confessing that she cheated. However, this was anticipated by Skinner, who fooled Lisa by staging a fake presentation before the real one, with the comptroller being revealed as Otto wearing a latex mask, allowing the school to keep the money anyway. Meanwhile, Homer discovers that he has accidentally cooked Pinchy while giving him a hot bath. Later that night, a distraught Homer eats his late friend, and finds him delicious.