When someone clogs the toilet, Lincoln investigates to find the culprit.
Lincoln is preparing for a comic book convention about superhero Ace Savvy, but when he prepares to use the bathroom, toilet water comes flooding out, showing that someone had clogged it last night. The girls think Lincoln did it, as he has a habit of doing so in the past, but he protests that he didn't do it this time. As usual, Lola tells Dad about it and he announces that they're all grounded until the perpetrator is found out.
With his trip to the convention in jeopardy, Lincoln decides that he has to find the culprit and Lucy willingly joins him in the mystery. First, they go to Lynn as she has eaten a lot of sub sandwiches the previous day, but she claims that she didn't do it since she believes that going to the bathroom is bad luck for her favorite roller derby team. Lynn suggests that they go see Lisa, who keeps a record of the family's toilet habits in a filing cabinet. Lincoln and Lucy find that the document for who used it between 1:00 AM and 4:00 AM is missing, making them suspect that is was Lisa, but she's vindicated when she admits that she prefers using Lily's training potty. They then suspect Lily since she's always throwing her diapers in the toilet, but she's proven innocent when Lincoln knocks over her diaper genie and spills out its disgusting contents. Lincoln then suspects Lori since she was the first to blame him for clogging the toilet, but an atomic wedgie from her convinces him otherwise.
Lincoln and Lucy try to get the truth out from the rest of their sisters, but all prove themselves innocent; Luan says that she was asleep at the time (which she proves by revealing that she records herself in case she makes a good joke in her sleep), Leni is too dumb to have done it, Lana says she's tried and failed countless times to clog the bowl, Lola says she would never get up to interrupt her beauty sleep, and Luna says she was at a rock concert that night (which she proves by showing a TV report showing herself crazily chasing Mick Swagger onstage so she can get a lock of his hair).
Meanwhile, Dad and Lana are fixing the toilet with the plunger "Big Bertha" where Lana discovers what clogged the toilet: a book titled Princess Pony, making Lincoln suspect Lola of being the clogger. He and Lucy go into her room to interrogate her, but she claims that not even she would read something so sickeningly sweet as that. Just then, Clyde arrives at the front door and hands Lincoln a page from the Princess Pony book (which Lana had thrown out the window earlier). Reading a quote from the page, Lincoln discovers that Lucy had said the exact same statement while interrogating Lola earlier, making him suspect that Lucy is actually the culprit.
Lincoln enters Lucy's room to get her to confess, but she denies being the reader. After a search through the premises, Lincoln finds Lisa's missing bathroom file in the overhead lamp and a poster on Lucy's wall flips down to reveal a Princess Pony poster behind it. Lucy confesses that she was the reader, something she tries to keep a secret from her sisters. She also reveals that the toilet got clogged when she tried to read it in the bathroom late at night. She hid it in the toilet when a half-asleep Lincoln came in to use it and he flushed it unaware that the book was in it. Lincoln demands that Lucy tell the girls the truth, but Lucy is nervous about it since they'll tease her more than they do him.
Lincoln and Lucy come downstairs to reveal to the girls the Princess Pony book that clogged the toilet, and they proceed to mock the book and vow to make whoever owns it a laughingstock. Just as Lucy prepares to confess that it was hers, Lincoln, feeling sorry for Lucy, claims that the book is his and takes the blame for clogging the toilet. As a result, Lincoln remains grounded and the girls are ungrounded, forgoing his chances of going to the convention.
Lucy asks why Lincoln took the blame, and Lincoln says he's comfortable with himself, and he has Lucy's back. Grateful, Lucy thanks her older brother for keeping her secret from the others. Later that night, Lucy cheers Lincoln up by presenting him with a creepy comic book she drew as thanks for helping her. The episode ends with Lynn going to the bathroom to relieve herself after winning the game and Lana calling for Dad to get Big Bertha again.