Bart and Lisa are watching television and they see a commercial for a personal diary, the Turbo Diary, which does not allow anyone but its owner to read it. If anyone else tries to open the diary, it gives that person an electric shock. Lisa, tired of Bart reading her diary, asks Homer and Marge to get her the Turbo Diary for her birthday and they agree. The next day, Homer and Bart go to the mall to get the diary for Lisa. Homer gets sidetracked by the loads of free food samples being offered and when they finally go to the toy store, all the diaries are sold out. On Lisa's birthday, Bart gives her a laser pointer. Homer gives Lisa a personalized film he made for her. She is disappointed that she did not get the diary she wanted, nevertheless she watches the film, which is a story about Lisa as a sheriff in a Western setting. The dialog in the film helps Lisa realize that Homer knows absolutely nothing about her. She runs upstairs crying, making Homer feel bad for her.
The next day, Homer gets an idea from Moe to hire Dexter Colt, a private detective, to find out facts about Lisa so he can appear to know things about her. Colt begins to spy on Lisa and build up information to produce a report. Using the report, Homer starts to bond with Lisa by playing the songs that she likes and going to a protest against animal testing at a research lab. Homer and Lisa's relationship is good again as she feels he does know her after all. Homer goes to see Dexter Colt to thank him for all his work. When Colt gives him a bill for $1000, Homer refuses to pay the incredible amount. Colt swears revenge as Homer runs off into the night.
The next morning, Lisa, Homer, and Bart are watching the news. They see that someone has vandalised the research lab and stolen all the animals. Chief Wiggum says that, based on the clues collected, Lisa Simpson is the main suspect. Lisa says she is innocent and Homer knows she has been framed by Colt. Homer and Lisa escape as a fugitives and disguise themselves. While on the run, Homer confesses about the report and Colt to Lisa, and she becomes angry with him once more. The police track Homer and Lisa to a motel but they escape again and find themselves at a circus. There they find all the stolen animals, and it seems Dexter sold them to the circus as a petting zoo and sideshow attractions. Colt shows up and chases Homer into a hall of mirrors. As Colt is about to kill Homer, Lisa shows up. Colt wonders how she heard them, and Homer says that Lisa's hearing is very good and that one day when she was very young, she had heard him quietly sobbing in the closet. Lisa, impressed that Homer actually remembered something about her, flashes Colt in the eye with the laser pointer Bart gave her, temporarily blinding him. Later, Colt is arrested, Lisa is exonerated, and the animals are given to Cletus and his family.
During the closing credits, it is revealed that Lisa finally got her Turbo Diary. When she goes to sleep, Bart sneaks in and steals the diary using a pair of tongs. While the house was shown, Bart is heard walking to Homer and asking him to read him a bedtime story. Homer accepts, and asks Bart to give him the diary. He opens it and gets electrocuted as a result of being an unauthorized user. The episode ends with Bart laughing at his gullible father's excruciating pain and misery.