After witnessing an excess of lying in the family, Lisa invents lie-detecting glasses to try and curb the fibs.
At the house, Lana requests to be driven to the dumpster by Lori, and Lori, not wanting to do that, lies by saying that she has homework to do. When Lana heads back up, Lori reveals that she's going to the mall. This act catches Lisa's attention. Later, Leni arrives home, with her hair dyed orange. The siblings pretend to love her new hair color, when they actually hate it. At dinner time, Lynn Sr. presents a cabbage casserole, and while he gets fish sauce, the family, who secretly hate the dish, discretely dispose of it all. These acts provoke Lisa, who, while in her room, realizes that if she wants the house to be a better place, she must come up with a way to stop her family's constant lying. Soon enough, she begins working on a new invention, which is revealed to be a new pair of glasses.
The next day, while everyone is eating breakfast, Luna and Luan are arguing over instrument use, when suddenly, Lisa, wearing her new pair of glasses, says that Luan has told a lie, and reveals that her new pair of glasses are built to detect lies. Throughout the day, Lisa uses her glasses to force her siblings to tell the truth, such as Lynn saying she trashed the bathroom, Lucy telling Rita that her novel isn't great, Luan telling Lily that her drawing is terrible, and Leni telling Lori that she used up all the hot water while showering. While getting a box of juice, Lisa is encountered by her enraged family members, where they demand her to stop using her lie-detecting glasses. Lisa denies, saying that she's "doing what's best for this family", and disagrees with Rita's statement that a little white lie can help smoothen things out. Realizing she won't stop, the family realizes there's one thing to do: destroy the glasses. At night, as Lisa sleeps, the family members sneakily get the glasses and manage to destroy them, as well as toss the remains out.
The next day, Lisa discovers the destroyed remains of her lie-detecting glasses, and the family feigns concern over its destruction. Lisa, knowing that they're lying, and because they want her to stop with the truths, reveals that she had a backup plan: she installed lie-detecting cameras all over the house that are protected from any sort of contact with protective shields. Throughout the day, the family is forced to tell the truth in any given situation, ranging from Lincoln telling Leni that her new haircut is awful, Lucy telling Rita once again that her addition to her novel is terrible, and Lori being forced to take Lana to the dump. At dinner time, Lynn Sr. once again presents cabbage casserole topped with fish sauce. However, the family, upset at having the tell the truth all day, tell Lynn Sr. that the dish is terrible, prompting him to toss the dish out the window in frustration, and instead gives them cereal. As the family eat their cereal in a huff, Lisa attempts to lighten the mood by asking Luan, Rita, and Lori for some interesting anecdotes, but because of how everyone feels about what they talk about, they all decline and leave, leaving Lisa all alone. Lisa still thinks that her family being angry at each other is worth them no longer being dishonest, but the cameras call her out for lying to herself. Eventually, Lisa admits that she has caused more harm than good, and realizes that there's only one way to fix it.
In the living room, Lisa calls for her family, and when they arrive, she says that while she still sees the truth as a good thing, she accepts that a little white lie can be good as well, seeing how it makes thing smooth. The family is happy that Lisa has accepted lies, and Lisa is prepared to have the cameras disabled. However, the cameras still have their protective shields, making Lisa get another idea. She instructs the family to go into different locations in the house, and orders them to tell various lies. Eventually, the family tells so many lies, the system overloads, and explodes, successfully disabling the cameras. Lynn Sr. decides to celebrate by making squid ink surprise, and Lisa, not wanting to tell her father that it's a bad dish, fibs that she approves. When Lynn Sr. heads to the kitchen, the other commend Lisa on getting used to telling lies.