When an attractive substitute teacher named Miss Felter takes over for Miss Slovak Arnold develops a crush on her quickly. He scribbles her name on his math homework, daydreams about her giving him a milkshake after being flattered by his homework, and is so distracted the next day that he misses the bus and runs several blocks after a car he thinks is Miss Felter's. At school, he believes he sees signs that Miss Felter returns his affections. She chooses to read the class a romantic poem written by Matthew Arnold, and keeps calling on Arnold himself. He becomes increasingly distracted by his fantasies, including one where Miss Felter hands him his A+ marked math homework in a wedding dress. He shares his crush with Gerald, who is skeptical until he overhears her talking about a dinner she has planned later that night for "Arnold."
Gerald dresses Arnold up for his supposed dinner, teaches him flirty lines, and tells him to watch out for signs indicating Miss Felter is serious about a relationship. He also provides him with a walkie-talkie in case he needs help. Arnold heads for Miss Felter's house. As the signs Gerald told him to look out for manifest (fancy napkins, lit candles and romantic music), Arnold gets increasingly nervous, and hides in the bathroom. Feeling he is in over his head, he begs Gerald to help him escape. The doorbell rings suddenly and in comes Arnold Skelter, Miss Felter's fiancé and the Arnold she really meant when speaking of her dinner. Miss Felter introduces the two Arnolds and says she and Skelter will be married in June. After acknowledging Arnold's misunderstanding, she tells him that she likes him nonetheless, and encourages him to continue being himself. Sad, but relieved, Arnold collects Gerald from the alley and goes home. Skelter is heard using one of the lines Gerald and Arnold had rehearsed earlier.