"Goes Cellular" is the sixth episode of season four and the forty-fifth episode overall of the animated children's series The Magic School Bus. It originally aired on October 18th, 1997.
It's the night Arnold is to receive the Great Rock Admirers' National Institute of Tectonics Experts' Rocky Award, which has never been awarded to a kid before. He's also mentioned being the youngest member of GRANITE. Ms. Frizzle and the class waits for him in the lobby of the Walkerville Hotel, but there's no sign of him. Carlos suggests maybe he ran out of Sea Wheedies and went to buy more. Ralphie, disgusted, says he can't believe he eats them. "He hasn't been able to eat anything else since he heard he was getting the Rock Award," Phoebe says. It's soon announced that everyone goes to the ballroom, to which Phoebe exclaims, "We can't go in without Arnold!" Soon, they hear someone whisper from behind a curtain. It's him, and he's completely orange, which he found out that morning, much to his horror. Dorothy Ann suggests they try to wash it off, so they go out to the parking lot and try to do so. Dorothy Ann wonders if the color is in his skin, rather than on it.
As Phoebe helps him put his shirt back on, he asks, "But how did it get there? How's my skin different from, let's say, Phoebe's?" This triggers a field trip, everyone but Arnold and Phoebe shrinking down with the bus. As Carlos, Tim, and Dorothy Ann explore the skin above Phoebe's left eyebrow, Keesha, Ralphie, and Liz are on Arnold's glasses. Ms. Frizzle and Wanda remain in the bus, which has become a decoration on Phoebe's headband. Ms. Frizzle shrinks those on Arnold and Phoebe down even more so they're at the cellular level. Meanwhile, Phoebe tries to keep Arnold calm, but a dog comes up behind them and takes his Sea Wheedies, making them chase after it. Realizing someone could see him inside the hotel, they both hide behind a potted plant. Phoebe gets an idea on how to hide the orange, and she grabs Arnold by the arm, pulling him into the kitchen.
As Keesha, Ralphie, and Liz try looking for a source of the orange in Arnold's skin, they're suddenly bombarded by white powder. It's flour. Phoebe covers Arnold's hands and face as they hide under a catering cart, but he wipes it off, arguing he looks like a pumpkin pie. That's when he remembers that Janet stayed with him and his parents a few weeks back. "I bet she made me orange!" he says. "It's just the kind of thing she would do!" That's when he notices a bowl of Sea Wheedies on another cart. On their skin, the rest of the class decide to go under it to see if the orange stops in Arnold's and if there's any at all in Phoebe's. Once they're there, Arnold and Phoebe's skin cells begin multiplying at the same time and push them out.
Arnold hears the head chef announce that dessert is almost to be served. He panics, telling Phoebe that the awards are after dessert. She hushes him and uses the Mega Magnifier to check on Keesha and Ralphie's progress with his skin. They tell her they don't know what's making him orange. "You guys aren't giving up, are you?" she asks, saddened. Keesha says that they had no choice due to being pushed out by the skin cells and says there's nothing else they can do. Ms. Frizzle turns the bus into a helicopter and picks up the rest of the class and Liz from Arnold and Phoebe's foreheads, and she lands on Arnold's last Sea Wheedie.
"I'm doomed," Arnold says. "And I'm down to my last Sea Wheedie." Seeing the bus, Phoebe yells out, "Arnold, no!" He tosses the Sea Wheedie in his mouth and asks, "You say something, Phoebe?" "Arnold, whatever you do, don't--" He swallows. "--swallow. The bus was on that Sea Wheedie. You just ate our class!" "Oh, no, not again!"
Inside Arnold's stomach, the rest of the class noticed it is more orange than the last time they were there, and then they see a fresh Sea Wheedie lose its seaweed coat, revealing an orange center. The bus follows it to his small intestine and into his bloodstream, and they realize that the beta carotene in the Sea Wheedies is what's making him orange.
Looking at Arnold's stomach, Phoebe wonders out loud where the rest of the class is, but Arnold says he doesn't want to think about it. He just wants to stop being orange. Suddenly, they're caught by the head chef. Arnold is soon called for his award, and he covers his face as he walks to the stage. In his bloodstream, Ms. Frizzle turned the bus into a "villi bus" to absorb all the orange, which works. Arnold's skin is changed back to normal as he steps onto the stage. He coughs out the bus and thanks, Ms. Frizzle and his class as he accepts his award. At the end, he learns it was the Sea Wheedies that turned him orange. Carlos ends it by asking, "Hey, Arn, orange you glad you aren't orange anymore?"
- This is the second time an animal takes something from Arnold and Phoebe, prompting them to chase after it, the first time being in "The Busasaurus."
- This is also the second time Arnold swallows the bus, the first time being in "For Lunch."
- Phoebe's eye color is hazel and Arnold's is brown, as seen in close-ups of their eyes. His are also brown on the Holiday Special DVD cover, but hers are green on the Creepy, Crawly Fun! DVD cover.