Mr. Simmons teaches the class about Monkeynucleosis, a made-up disease that a person gets from contact with a monkey. After Helga comes in contact with an organ grinder's monkey, she makes herself believe she has the disease. Her arm forms a rash and she decides she needs to check it out. Helga takes a book from the library on fake diseases. She finds Monkeynucleosis and as the episode goes on, she finds she has all the symptoms.
First, Helga reads that the first symptom is itching and puffiness. She knows she has that. The next is sweaty palms, then loss of appetite, then irritability, and finally expiration. She walks away with the book and says "Well, at least I've only got one of the symptoms." at that moment the book slips out of her hands. Helga gasps and knows she has the second symptom.
As she walks home she hears people screaming things relevant to monkeys.
Later, at dinner, Miriam asks Helga why she isn't eating. She replies by saying she has no appetite. Helga remembers the third symptom was loss of appetite. Bob sees her book and reads Monkeynucleosis. He calls it a 'crazy ol' made up disease. Helga asks if he thinks its real and he says he doesn't know.
That night, Helga dreams that she is half human and half monkey and is being shown to an audience. They gasp in disgust and call her an animal. The audience throws food at her and she tells them shes not an animal, she's a girl. She runs outside. As she is charging down the street people scream with horror when they see her. She sees Arnold and tells him to wait, he replies by saying "Sorry, do I know you?" she tells him she's Helga, and that she loves him. He says she's not Helga, she's a monkey girl. He fades into the fog, and leaves Helga crying. the organ grinder from before comes and says "Where have you been?" Helga doesn't know what he's talking about and he says the people are waiting. he dresses her up and puts a cup in her hand. When he plays she starts dancing and has no control of herself. A woman yells "Can't she do anything else?" which is exactly what Helga had said while watching the real monkey dancing.
Helga wakes up because Phoebe calls asking what their math assignment was. Helga says angrily that she doesn't know. Phoebe says she doesn't have to be so irritable about it, and Helga thinks she has the fourth symptom. (It is important to note that Helga is always irritable.) Helga tells Phoebe to bring all the kids to her house because she thinks she is going to expire.
Helga gives Phoebe her books and asks her to return her library book on old diseases. Phoebe looks at the book with curiosity, and in the next frame it shows Rhonda with a few of Helga's dresses. Rhonda comes downstairs and tells Arnold that he's next.
Helga starts to tell Arnold why she's called him names and tortured him, she says that she doesn't really hate him. When she's about to confess, Phoebe bursts in and says she doesn't have Monkeynucleosis. Arnold asks Helga what she was going to say and Helga says she was delirious. She insults him and he starts to leave. She tells him to wait and says she thinks he's ok. He puts his hand on her arm and thanks her, saying she's OK too. When he leaves she says happily that he thinks she's OK, and he touched her, and she's not a monkey.