Lincoln and Clyde are making chocolate pies for a business, but when Lincoln goes to get chocolate for the pies, he notices that all the chocolate is gone--Lincoln immediately deduces that his sisters, who're all notorious "chocoholics", must have eaten the chocolate. Since they can't make chocolate pies, they decide to make salt-and-vinegar pies instead (stating it works on potato chips). Artie tastes a pie, and claims it to be disgusting, making them switch to birdhouse making. The birdhouses don't go much better, because when a bird flies into it, it immediately collapses.
Then they see Luan has earned $100 by selling balloon animals, causing them to take up the idea. While Liam is looking at the horribly misshapen balloon animals and explains that he's trying to impress a girl he likes, Lincoln suggests that Liam should give his crush some chocolate, revealing that his sisters all love chocolate. Liam takes the advice, and it ends up working, which leads to Liam calling Lincoln a "Girl Guru" ("guru" essentially meaning "expert"). This gives Lincoln and Clyde the idea to start a business by having Lincoln give girl advice to people for money.
Lincoln gives people advice based on his past experience with his sisters. Later, he and Clyde see the boys trying Lincoln's advice, which ends in the girls hating the boys for doing such. This causes all of them to hate Lincoln and Clyde, vowing to cause them pain the next time they see them.
Lincoln and Clyde then leave the school and run home hiding in trash cans. When they walk inside, the sisters immediately question what they were doing. Lincoln explains that since he had ten sisters, he thought it wouldn't be a bad idea to use his living experience with them as advice to give to boys on how to impress girls. The moment he's done explaining himself, the girls end up laughing hysterically at the idea, pointing out that having a bunch of sisters doesn't make him a girl expert, adding that all girls are different and that what works for them might not work for other girls. Lincoln then wonders what to do now, knowing that his business is a complete bust. When he hears his father say something about tossing out Lincoln's salt and vinegar pies, he gets an idea.
Lincoln and Clyde are now selling the salt and vinegar pies to people as a way to get revenge on the "Girl Guru" himself. Of course, every one of their dissatisfied customers purchase a pie, and immediately pelt Lincoln with them.