After Homer has Lisa rummage through a gourmet grocery store freezer for ice cream, he purchases a small lobster, intending to fatten it up to eat. When they get home, Lisa has developed a cold. Marge makes her stay home from school and has her play Bart's Dash Dingo video game to pass the time.

When she returns to class, Lisa realizes she forgot to do the reading assignment Ralph brought her, just as Miss Hoover ditributes a test. Desperate, Lisa fakes ill again. In the hall, Lisa runs into Bart. When she tells him of her predicament, he brings her to Nelson, who offers to sell her the answer sheet. Lisa is hesitant, but decides to buy it. Armed with the answers, Lisa goes on to score an A+++. Meanwhile, Homer balks at boiling up the lobster. Instead, he names it Mr. Pinchy and adopts it as a pet.

Lisa's test score raises Springfield Elementary's average, qualifying the school for a large state grant. Feeling guilty, she tells Skinner she cheated. He and Superintendant Chalmers ask her to tell no one so that the school will qualify for the money. Lisa agrees, but at the check presentation, she tells the school the truth. She's praised for her honesty and then she leaves. With Lisa gone, Skinner and Chalmers reveal that the ceremony was a scham and then hold the real ceremony in which the state comptroller gives them the money. At home, Homer realizes he has accidentally cooked Pinchy by giving him a hot bath. He sadly eats, and thoroughly enjoys, his late friend.