Note: I do not own South Park or any of its characters. Oh, and spoilers for You're Getting Old and probably future episodes. I hope Stan does come back but this is assuming he's been gone for three years.
"Stan!" Cartman exclaimed rushing into the bathroom after Stan. "What's wrong?" There was a farting noise in one of the stalls. "I-"
"Stan?" Kyle asked, following Cartman. "Stan, what's...ew!" He wafted his hand in front of his face as there was another farting noise accompanied by a smell.
"I don't...oh!" There was a third noise. "I don't know, I just- uggh."
"Don't worry, Stan." Cartman assured. "I'm sure-" There was another noise and Cartman took a step back. "Well, uh...maybe we should take you to the doctor's."
"No, I just- Ohhh!" Stan groaned. "I just need to stay here until...ahhh..."Stan sighed in relief. Cartman smiled. Over before Stan'd suspect anything. "I feel better now."
"Are you sure?" Kyle asked.
"Yeah, let's go back to watching the movie." Stan said and there was a rustle of fabric as he pulled up his pants and exited the stall.
Cartman hid his disappointment. "Are you sure that's a good idea?"
"Yeah, dude." Kyle seconded. "I mean you seem pretty sick."
"Yeah." Stan said. "I feel better now. Let's-oh!" He dashed back into the stall.
Stan didn't know what was wrong. He'd been fine this morning! Maybe there was something bad in his popcorn or…his soda! Damn, he'd let Cartman have some and then when Stan drank from it he'd gotten sick! "Cartman…" He growled.
"Yes, Stan?" The boy's voice came.
Stan was about to tell him off but then he thought better. He couldn't prove that Cartman had put something in his drink and Kyle would just think Stan was being paranoid. He sighed. "Cartman, Kyle, maybe you should just finish the movie without me. I think I'll just go home."
"No, Dude." Kyle answered. "You're too sick to be alone."
"Yeah, we'll walk you home." Cartman seconded.
"No, really." Stan moaned as he was sick again. "I can't- ohh!"
He was like that for the next ten minutes but neither Cartman or Kyle left. When Stan could finally pull up his pants and leave, they walked him out of the theater and down to the motel.
"You stay here, Kyle." Cartman said, when they got there. "I'm going to make sure Stan gets up to his room."
"I'll go with you." Kyle disagreed.
"No, that's okay, Dude." Stan spoke up. "It'll just take a moment."
"Okay." Kyle answered reluctantly.
"I'll be right back down." Cartman assured. "Wait for me?" Kyle nodded and Cartman and Stan headed into the moment the doors had closed, Stan stabbed the button and backed away from Cartman. "Whatever is the matter, Stan?" Cartman asked innocently.
"Dude, you can…just drop it okay?" Stan snarled.
"I know you did something to my drink so you can just stop acting all nice. It's just the two of us, so admit it."
A slow smirk spread across Cartman's face. "Stan, I know you're jealous that Kyle likes me better than you but-"
"He doesn't!" Stan yelled, causing Cartman to grin even more. "He likes who he thinks you are! Once he finds out that you haven't changed he'll go back to hating you!"
"Oh, but I have changed, Stan." Cartman sneered. "Kyle and I are best friends now and I'm not about to let you get in the way of that."
"You won't have a choice, you asshole." Stan replied. "I'm planning on hanging out with you guys for the rest of the time I'm down here. Sooner or later I'll find a way to let Kyle know you're not as nice as you act."
"And how will you do that, Stan?" Cartman chuckled. "Tell him I poured laxatives in your drink? Tell him I'm still a manipulative asshole?" He shook his head. "Stan, Stan, Stan. You've been gone for too long. Kyle trusts me more than you now. I suggest you get used to it." There was a ding and the doors slid open. "You're floor?" Cartman gestured towards the exit. Shaking with rage and unable to think of a good response, Stan leapt forward, stomped on Cartman's foot, and raced off toward his room.