After witnessing an excess of lying in the family, Lisa invents lie-detecting glasses to try and curb the fibs.
At the Loud 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, her hair dyed orange. The siblings pretend to love her new hair color to her face, but then express that they actually hate it behind her back. At dinner time, Lynn Sr. presents a cabbage casserole, and while he gets fish sauce for the topping, the family, who secretly hate the dish, discretely dispose of it all. These acts provoke Lisa, who, while in her room, theorizes 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.
The next day, while everyone is eating breakfast, Luna and Luan are arguing over the latter borrowing the former's gong without permission, which Luan denies. However, Lisa, with her new invention, says that Luan is lying, and reveals that her invention is a pair of glasses that are built to detect lies. Throughout the day, Lisa uses her glasses to call out her siblings' dishonesty, such as Lynn denying that she trashed the bathroom, Lucy telling Rita that the newest addition to her novel is good when it's not, Luan telling Lily that her drawing is nice when it's just scribbles, and Leni denying to 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, who demand her to stop using her lie-detecting glasses. Lisa refuses, saying that she's "doing what's best for this family," and disagrees with Rita's statement that a little white lie can sometimes 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, tossing the remains in the trash.
The next day, Lisa discovers the destroyed remains of her lie-detecting glasses, and the family feigns concern over their destruction. However, an alarm sounds and a computerized voice calls them out on their lies; it turns out that Lisa, having anticipated that her family might resist her attempt to enforce total honesty, 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 among other things. At dinner time, Lynn Sr. once again presents his cabbage casserole with fish sauce. However, the family, upset and grumpy at having to tell each other the truth all day, tell Lynn Sr. what they really think of his dish, prompting him to toss the dish out the window in frustration, and instead give them cereal. As the family eats 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 refuse. Eventually, tensions boil over to the point where everyone leaves to eat in their rooms, 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 concedes 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 admits that while she still sees the truth as a good thing, she had accepted that a little white lie can be good as well, seeing how it makes things 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 others commend Lisa for getting used to telling lies.