"Along Came a Sister" is the twenty-third episode of The Loud House.
The story starts at Lincoln's school, where he, Clyde, and their classmates are looking at their pet tarantula, named "Frank". When class ends, Mrs. Johnson asks who will volunteer to look after Frank for the weekend. Everyone raises their hand, and she choose Lincoln, much to his glee. However, Mrs. Johnson says it's a big responsibility to look after Frank and he must not be let out of his cage, unlike herself this weekend, much to the confusion of her students.
After school, Lincoln says he's excited to be the one looking after Frank but Clyde says to keep him far away from Leni, who greatly fears spiders. Clyde flashes back to when he came dressed as a spider to the Loud House last Halloween, with Leni answering the door and panicking when she thinks he's a real spider where she sprays him and flees the house. Clyde says that Leni didn't come out of her room for three weeks. Lincoln say to the viewers if he has to worry about his ten sisters every time he wanted to do something, he would never be able to do anything.
Lincoln decides he will use stealth to get Frank past Leni and the others. Inside, Luan chases Leni with a fake spider while Lincoln hides the tank with a trench coat. He gets startled by Walt, then runs upstairs and covers Walt's cage with a hoodie. In his room, Lincoln greets Frank when Lana invites herself in and ask Lincoln for scissors, as she holding a doll, but gets excited when she sees Frank. Lana asks if she can play with him, but Lincoln says no and says he has to stay in his cage. An angry Lola rants that her twin stole her doll and wants it back until she sees Frank and finds him to be adorable. They ask if Leni has seen him, which Lincoln says she hasn't and to keep things quiet. Then Lisa comes in to say she's trying to do math and, like the twins, she's also fascinated by Frank. Lincoln asks her to keep him secret only to find his other sisters (Lori, Luna, Luan, Lynn, Lucy, and Lily) are all interested in Frank as well, much to Lincoln's annoyance. Luna asks if Leni has seen him and Lincoln for the final time tells them no. Then Leni comes in, causing her siblings to panic and block her view of Frank. Leni is confused as to what they are hiding, but luckily she thinks that her siblings are throwing a surprise party for her, so she leaves. They sigh in relief, leaving Lincoln to keep an eye on things.
Lori tells Lincoln he better knows what he's doing, as he smiles at her nervously. Lincoln sees that Frank seems tired and he let's him out of his cage. Before he can feed him some crickets, he sees Frank has escaped. He panics and searches everywhere in the house, throwing off his sisters by saying Lana and Lola are the cutest twins, ruining Lisa's experiment and telling Lynn to freeze, making her think they're playing Freeze Tag. In Luna and Luan's room, Lincoln is searching all over when he spots a spider in the girls' closet. He goes after it, only for Luan to reveal it was one of her fake ones.
Lincoln tries looking in the bathroom, prompting Lori to demand to know why he was seemingly peeping in on her. He quickly makes up the excuse that he was looking for his contact by sticking a lint ball in his eye. He runs past Lynn (who is still frozen) to his room and panics over losing Frank. His sisters confront him and he admits he lost Frank, much to their frustration, until Leni eavesdrops on them and asks what they are whispering about. She still thinks they are throwing a surprise party for her and once her back is turned, it shows that Frank is on Leni's back, much to the horror of the others and they all run downstairs. As Leni goes to make herself a smoothie, everyone sees Frank is missing. Leni looks in the fridge to get milk, and they see Frank on the carton. Lisa quickly closes the fridge and tells Leni that she is lactose intolerant, which simply confuses Leni. When she opens the fridge, Frank is not there, much to their relief. Lily then spots Frank and Luna traps him with a pot and they pretend to play drums to throw Leni off.
Frank goes missing again and Lincoln spots him on the ceiling. Leni sees him and thinks it's one of Luan's fakes until she sees that Frank is real. Terrified, she sprays Frank before her siblings can stop her and she flees the room saying their surprise party was terrible. Once the smoke clears out, Frank is dead, much to the sadness of the siblings. Outside, a funeral is held and Clyde comes to bring a casserole for Lincoln. Lincoln laments that his class is gonna hate him. The service is held and the rest of the sisters start blaming Leni for what happened, but Lincoln comes to Leni's defense by pointing out that it was his own, since he brought Frank home and let him out of his cage despite knowing about Leni's arachnophobia. While washing her hands, Leni overhears this as they all mourn Frank.
Cliff then coughs up a hairball, much to Lola's disgust. Lana then see that the hairball looks exactly like the one in the shoe box. Lincoln sees that "Frank" is actually a hairball, meaning he may still be alive, and then they hear their father screaming. Clyde recognizes it as their father's as he also got trampled by him on Halloween when Dad saw he was a spider, which Luna finds to be hilarious. An exterminator pulls up and they quickly go out front. The exterminator kills two caterpillars and goes inside. Lincoln say he's gonna look for Frank and everybody wants to help distract the exterminator. Lincoln accepts and stops to unfreeze Lynn before the two siblings run into the house. During the search, the exterminator is outsmarted by the siblings and Clyde. Lincoln finally sees Frank upstairs and both of them make a run for it. The exterminator corners Frank and finally sprays him, much to everyone's shock and dismay. Suddenly, a figure in a hazmat suit appears and everyone is shocked when it's revealed to be Leni, who saved Frank at the last minute, and shows him to be safe in her hands. Everybody cheers her on.
Leni is furious with the exterminator, and explains that he'll destroy Lincoln's reputation if he kills Frank. The exterminator is touched and tells the two that they changed his life forever, but actually made that up and storms out angrily, leaving the family their bill. Lincoln praises Leni and she explains that she heard what Lincoln had said at the funeral and seems to have a change of heart about spiders. She stops to admire Frank, who she admits is kind of cute, but changes her mind when he blinks at her, causing her to panic and run away. Lisa comments on how unusual Frank's name is--according to her, Frank's actually a female spider (as told by some markings on her) and that the reason she seems sluggish is because she was to give birth soon, much to everyone's delight.
The siblings say their goodbyes to Frank (renamed Frances) as Lincoln takes her back to school. However, while Leni is doing her hair in her room, an egg sac from Frances hatches in the vent above that produces countless baby spiders. Leni screams in horror, and Lincoln stops to realize the trouble he's now in, as the episode ends.
Penelope and the other classmates have no lines in this episode.