This is my first story, a one-shot. I was just thinking about how the Dai Li brainwashed Jet when they captured him (and also Joo Dee(s), but they weren't captured I think). So what about Appa?
This takes place after Lake Laogai.
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Brainwashed Appa
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"Aang? I'm not sure that Appa is okay."
Aang turned around to face Sokka. "Why shouldn't he be? He's back from the Dai Li!"
Sokka scratched his head, and explained his thoughts. "You know how the Dai Li keeps brainwashing everyone? All those Joo Dees, and Jet when they captured him?"
"Yes…what does this have to do with Appa?"
"Well, why wouldn't the Dai Li brainwash Appa too?"
Aang shook his head. "No…He's a Sky Bison! You can't brainwash Appa!"
"Well prove it!"
Aang turned and walked over to Appa, who was munching on some hay. "Okay, fine. Look. Appa, the Earth King has invited you to Lake Laogai."
Appa blinked, turned his head toward Aang and snorted, before returning to his hay.
Aang smiled, and returned to Sokka. "See? He's fine. Nothing wrong."
Sokka smiled too, saying, "Just making sure."
It was getting late, and the Gaang went to bed. But as they were sleeping, a low voice moaned across the camp.
"I am honored to accept his invitation…"