Rhonda has a party and only invites the cool kids. but when Arnold attends, he realizes that her party is only for certain people.
Rhonda holds a party for only cool kids, and Arnold's invited---but almost every other kid is on a geek list, which upsets them. Feeling bad, Arnold considers not going, but everyone tells him he should go anyway.
After going to Rhonda's cool party, Arnold shortly leaves because all they do there is listen to Rhonda's stories. After leaving, he's placed on the geek list. Arnold decides to hang with the rest of the kids and sees them throwing rocks at a dumpster can. Arnold explains that the party was really lame, but the other kids are still mad at Rhonda for not inviting them to her party, and come up with ways of ruining it to get back at her---Curly even suggests painting themselves in tiger stripes and freeing all the animals from the local zoo. Helga tells him that he go on ahead and they will meet up with him, just to get rid of Curly.
However, Arnold decides to hold a "geek party" on the roof of the Sunset Arms boarding house. When people start hearing about Arnold's party and how much more fun it is, people end up leaving Rhonda's "cool party" (and the cool list) to be at Arnold's geek party---and Rhonda puts them on the "geek list." Arnold's party proves so much more popular than Rhonda's that even the older kids of their school and neighborhood, including 5th grade bullies Wolfgang and Edmund, popular 6th grade girls Connie and Maria, and even Stoop Kid decide to attend. Eventually, even Rhonda's best friend, Nadine, leaves the party for Arnold's party, and Rhonda ends up putting her on the geek list. Before she leaves, Nadine tells Rhonda that the party was pretty lame, and also points out that since Rhonda crossed everyone off the cool list and put them on the geek list, Rhonda's not just the only "cool kid" left at the party, she's the only person left at the party.
After Nadine leaves, Rhonda realizes that her party was pretty lame, and eventually goes to Arnold's geek party. Rhonda proclaims that she wants to be a geek, but only for tonight- since the party is only for geeks. The party members welcome her and the party goes on. At the closing of the episode, it is revealed that Curly actually did free zoo animals, as we see him on a giraffe (where he painted himself orange with black stripes like a tiger) and followed by a pair of zebras running on the streets outside of Arnold's house, with Curly's trademark maniacal laugh.
This is the 100th eleven-minute episode of Hey Arnold!, and was made as a "reunion" show featuring all the kid actors who worked on the series. Also; according to Craig Bartlett, the scene where Arnold shows Rhonda that he's wearing a plaid shirt, not a skirt, was a reference to the fans mistaking Arnold's shirt as a skirt.
Rhonda's Cool and Geek list (partial segregate)
This list in the image on the right, and transcribed below, is likely only a part of the full list because it misses several characters such as Helga and Phoebe.