While Lincoln is playing video games in the living room, Leni arrives, looking for a new chapter book to read after Lily took hers to make a book fort. While looking under the couch, Leni finds a purple book with a guitar, and upon reading the names listed (which include Luna, Luan and Lincoln), Lincoln discovers that Leni found Luna's diary. For the sake of her privacy, they suggest that they put it back in its hiding spot, but the two are tempted to read what Luna's big secret is. However, before they could read any further, Luna arrives, prompting them to hide the diary, and upon entering, Luna takes her diary from her hiding spot and heads upstairs. Now more desperate to find out what Luna's big secret is, Lincoln and Leni resolve to find out.
The next day, Luna heads out to band practice with Sam, and Leni, under the guise of picking up laundry (with Lincoln hiding in her basket), sneak into Luna and Luan's room to find the diary. Suddenly, Luan enters and interrogates Leni into revealing why the two are sneaking in their room. When Leni says she and Lincoln are trying to find Luna's diary, Luan is appalled that they're trying to invade her privacy, but is tempted to look by Mr. Coconuts. After some searching, Leni finds the diary under Luna's bed sheets, and reads that her big secret is that she skipped school to see a movie with a girl named Roxy. Thinking that Luna is becoming a bad girl, the three decide to keep a close eye on her.
Throughout the days, Lincoln, Leni and Luan learn more secrets about Luna, like how she got a tattoo on her ankle and sneaked to an underground club with Roxy instead of studying for finals. While spying on Luna from Lori and Leni's room, Luan enters and says that the latest entry in the diary reveals that Luna and Roxy are planning a night of mayhem in Royal Woods, where they plan on filling the fountain with nacho cheese, release all the goats at the petting zoo, start fights at a biker club, and steal the "R" from the Royal Woods Town Hall. Wanting to save Luna from being a bad influence, the three decide to foil their plans.
Later that night, Lincoln, Leni and Luan, donning ninja suits, attempt to beat out Luna and Roxy at their intended destinations. Traveling by bike, they first arrive at Flip's Food and Fuel, where they proceed to buy all of his nacho cheese. Next, they arrive to the petting zoo, and attempt to relocate the goats into a nearby barn. When the goats suddenly run rampant, Lincoln quickly calls Liam and tells him that the goats just need something to lure them, so they use the cheese, and hide them just as Luna and Roxy arrive. Next, they arrive to The Biker Brawlhouse, the biker club where Luna and Roxy are expected to start fights. The three decide to shut off the sign, but T-Bone, the Biker Woman, and the other bikers chase them away for turning it off (and for Luan insulting their body odor).
Lastly, they head to Royal Woods Town Hall, only to find out that Luna beat them to it and is preparing to take the "R" off the building. When they spot a security guard making the corner, the three come out to warn Luna, and ultimately confess that they read her diary. Luna is understandably furious that they invaded her privacy, but Lincoln, Leni and Luan were only trying to help her from becoming a bad person growing up. Suddenly, Luna laughs, revealing that she actually knew they were reading her diary. She explains that on the day Lincoln and Leni found the diary, one of Leni's strands of hair was stuck to it and overheard the two stating that they read it. This gave Luna the idea to write fake stories in order to mislead them, and her and Roxy's "night of mayhem" was entirely a ruse to get them to confess, with Roxy revealed to be Sam in disguise the entire time.
With everything revealed, Lincoln and Luan apologize for doubting her, and Luna apologizes for the trouble she put them through, but the moment is cut short when the security guard patrolling the building (who is actually real and not an actor Luna hired as Leni had mistaken) chases them off. After escaping, Mr. Coconuts attempts to ask Luna what her big secret really is, but Luna, annoyed and not wanting to tell, throws him in a garbage can.
Smarty Pants Challenge
During the initial airing of this episode, the viewers had the chance to answer trivia questions related to the episode with the Nickelodeon Screens Up app or on Nickplay.com for a chance to see their name on screen.