On the bus Helga is drawing a picture of Olga and stabbing it with a red crayon. Arnold sees this and Helga tells him that Olga is coming home for two weeks and every time she comes home, her parents completely forget about her. Arnold thinks that maybe this visit will be different and they can finally get along, but Helga thinks otherwise, saying nothing will change between them.
At her house, Big Bob and Miriam place a banner for Olga and when she opens the door they say "Welcome Home!" only to find out it was just Helga. Then Olga opens the door, accidentally hitting the door against Helga. Big Bob and Miriam give her hugs and tell her how much they've missed her. Olga then sees Helga and gives her a kiss. She then announces to her family that she joined the Big Sis Lil Sis program where she will be a big sister to an only child. Helga is initially delighted by this news because she knows that Olga will be preoccupied with someone else and will leave her alone.
Olga cooks up a fancy dinner and invites her "little sis" for a slumber party. Then Helga finds out that Olga's "little sis" is Lila Sawyer. After dinner, they watch really sad old movies. Olga says that it reminds her of a class she took in college where she watched really old sad movies and felt bad, but found away to make herself happy by dancing around, which Lila likes to do as well. They go upstairs and have a tickle fight, which disgusts Helga. Olga then says that Lila is the best little sis she ever had, shocking Helga.
On the bus, Helga tries to shoot a spit ball at Lila, but hits Arnold instead. She tells him how Olga and Lila are spending time together, which drives her nuts. Arnold thinks that Helga has become jealous. Helga tries unsuccessfully to cover her feelings, and Arnold advises her to tell Olga how she feels. But Helga disagrees and tries everything she can think of to mess things up. First she steals Olga and Lila's opera tickets, but a nice old lady had extras and invites them to sit at her private balcony. Then Helga takes out motor oil in Olga's car and when they are stranded, a nice family comes and helps them and serves them breakfast. Then Helga loosens a screw from a horse carriage. However, Lila is able stop Buttercup,the horse, by using experience she gained working with animals while living on her farm. Helga is angered that her plans aren't working. She sees Arnold again, and he tells her that she needs to tell Olga how she feels. Helga keep on telling him that she is not jealous, but he points out that if she isn't jealous, why can't she just leave Olga and Lila alone?
At home, Helga sees a cake and a picture and hears music that reminds her of Olga and Lila. She then approaches Olga and admits that she is jealous of the relationship she has with Lila. A touched Olga then reassures Helga by telling her that she will always be her "baby sis" and that they have a strong "bond" that only real sisters have, and that nobody can break it. This makes Helga smile. Olga then invites her to come moonlight sailing with her and Lila. Helga accepts the invitation. During the moonlight sailing, Helga is stuck rowing the boat and though already annoyed, she becomes increasingly agitated as Olga and Lila start to sing. Helga then decides to jump out of the boat and swims away unnoticed. 15 seconds after Olga and Lila finish singing, they notice that Helga has disappeared and Olga says "Oh my. Where's Helga?" and the episode ends.