The siblings are holding a meeting, but Leni isn't present, due to Cliff distracting her. The siblings are discussing Rita's upcoming surprise party, and Lincoln explains to the viewers that the reason why Leni isn't present is because she's notorious for spoiling surprise parties. After Lori gives a rundown of what each family member's task is (Lori will clean up, Lola and Lynn will make the food, Lana and Lucy will decorate, Luna and Luan will provide entertainment, Lisa will handle the guest list, and Lynn Sr. will take Rita out of the house), she informs Lincoln that he has the most important task: keeping Leni occupied while everyone gets the party ready. Without warning, Leni enters the room, and Lincoln tricks Leni into asking him "urgent" fashion questions, while their remaining family members sneak out.
When Lynn Sr. takes a clueless Rita out to go shopping, the sisters get to work prepping everything up. However, when Leni decides to go downstairs to get something to drink the sisters quickly try to hide their decorations so Leni doesn't see them, with Lincoln unable to warn his sisters in time due to forgetting their code phrase for Leni. After having shown Lincoln every type of fabric and pattern, Leni prepares to gossip with Cliff, but Lincoln once again distracts her by letting her use him as a model. In the hallway, Luna and Luan attempt to bring their entertainment equipment downstairs without making too much noise, however, Luna trips on a set of wind-up teeth, causing her to drop her drums, catching Leni's attention. Lincoln tries to warn Luna and Luan about Leni, but in the process, accidentally falls down the stairs. When Leni checks up on Lincoln, Lincoln comes up with a way to fully distract Leni: go to the Hazeltucky Party Outlet to get a scratch-and-sniff card that features a camel wearing a spacesuit that says "Happy Birthday from your 11 kids!" Falling for the trick, Leni leaves, thus giving the remaining siblings enough time to set up for the party.
As the siblings prep things up, they soon realize that the party is disastrous because Lana has to resort to using chewed up gum as an alternative for balloons, Lucy has to use toilet paper when she runs out of streamers, Lisa has invited Flip, Mr. Grouse, and Dr. Feinstein, saying they're the ones who Rita frequently interacts with, Luan's jokes revolve around her making fun of their mom's age, Luna's rocking music is ear-grating, Lola's cake is too sweet, and Lynn's subs are too spicy to handle. As the siblings begin to ponder what they did wrong, they realize that they need Leni, because despite her spoiling the surprise, she actually knows each of her family member's interests. Lori tries to call Leni on her phone, but Leni mistakes her ringtone as music coming from the selection of birthday cards present. Because of this, the siblings proceed to look for their sister themselves.
Several hours later, the siblings, who were unsuccessful at finding Leni, arrive back home. Upon entering, they discover that the entire living room is decorated, and there's a more refined guest list, such as Albert, Pam, Ruth, and a brunette woman. It turns out that Leni arrived back home while they were away, and thought even if they were going low-key with their Mom's party, she wanted to decorate away (she even managed to get the card that Lincoln requested). Suddenly, Rita and Lynn Sr. arrives back home, prompting everyone to find a hiding spot. Upon entering, the guests shout out "SURPRISE!", and Rita is happy about everyone's hard work. The siblings apologize to Leni for leaving her out and decide to make her the family's official party planner, however, Leni ultimately spoils an upcoming surprise party for Lynn Sr., causing her family and the guests to facepalm out of exasperation.