Disclaimer: I do not own Dragon Ball, or any of its associated characters and lore. That right belongs to Akira Toriyama.
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Goku Gets Trapped in the Demon Realm
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Outside the Demon Realm Door, Fiend Village, Earth, Age 750
With all his natural strength, Goku pulled hard on the golden sword keeping the portal to the Demon Realm open. After rescuing Princess Misa, he had no intention of the approaching Demons coming back out to plague the local villagers ever again, even if he would be happy to fight them more. Finally, he was able to pull the sword out, but before he could run from the Demons before the door closed on him, a fast pacing Demon, who Goku recognized as their leader, Shula, grabbed him by the tail and pulled him back into the Demon Realm as the door closed on them.
"Oh, no!" Princess Misa cried out in concern.
"Goku's been trapped in the Demon Realm." Her father, King Kress, sadly echoed her thoughts.
"Father!" Misa spoke to him worriedly. "What can we do? Isn't there anything we can do to save him?"
At a loss for words, the King eventually said. "I don't know, my sweet. This situation might be beyond our power to resolve." He then thought back to when he first met the boy and spoke again with some strength. "I'll send a message to his martial arts teacher, Master Roshi. He deserves to know, and he might be aware of a means to save Goku that hasn't come to me at the moment."
Turning back to the sealed door, Misa sighed with sadness, internally blaming herself for the predicament. "I can only hope Goku does pull through until then."
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The Demon Realm
Melee and her fellow Demon, Gola, looked on in dismay as Shula held tight to Goku's tail, the young fighter punching and kicking in an attempt to free himself. Having had enough of Goku's resistance, Shula threw him to the wall on the left and scrutinized his two double-dealing subordinates.
"Do I dare ask why you two kept me from my bride?" Shula remarked with great annoyance.
Stepping forward, Melee spoke first. "It wasn't our intention to betray you, Shula. Gola and I only meant for you to learn humility and to not overstep your boundaries regarding passage into the Living World."
Gola then said. "Besides, I know that everyone has their own tastes, but the Human Princess just wasn't developed yet for marriage." Seeing Shula's glare at him, he quickly added. "But if it soothes your anger, please let me take full responsibility."
Before Melee could speak in her comrade's defense, Shula sighed at last and relaxed. "Well, I suppose that I had to taste defeat eventually." Making a quick chuckle, he then said. "And there's no sense for a good party to go to waste." Seeing his subordinates relax as well, Shula then turned to Goku when he made his presence known again. "How about it, kid? Want to come along?"
With his stomach grumbling, Goku sheepishly replied. "Well, I can always use a bite to eat, or several." He then looked at Shula curiously. "But if you're going to forgive your friends, where do you and I stand? I'm not really interested in grudges, but it looks like I'm stuck here." He then had a cheerful look as if he remembered something. "Unless my friends wish me back home with the Dragon Balls!" His happiness soon deflated when he then took out a circular stone from his clothing. "Uh, never mind."
Shula chuckled, but much to Goku's surprise, without any known malice as he said. "Don't overthink it, kid. It's nothing a good spar between us can't resolve. As for you being stuck here as you said, I do know of some means to get you back into the Mortal Universe, but it will take some time. Maybe even years."
Thinking it over, Goku soon replied. "Well, I'll trust you'll find a way, Shula. And it would be an honor to fight with you some more." He bowed to Shula with respect before saying with concern. "But I kind of wish that my friends know I'm alright."
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Kame House, Three Days Later
As Master Roshi observed his students continuing their training, he saw an approaching bird carrying a letter arriving at his island home.
"Huh, wonder who that could be." He remarked as he then got the letter and read it to himself with initial curiosity that ending with shock and dismay. "Goku's trapped in the what now?!"
"What's wrong, Master Roshi?" Launch said to him in concern.
Turning to everyone present, Roshi called out. "Listen up! We got to make some calls to Bulma, my sister Baba, Korrin, and anyone else who can help us save Goku from who knows what kind of torment lurks in the Demon Realm!"
"Huh?!" The gathered people commented in confusion with some worry added as well.
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I'm sure those of you who saw episode 81 of the original Dragon Ball anime series know where this one-shot's inspiration came from. I can only imagine the kind of lasting effects this will have on the larger story from the rest of the first series to Z and so on so forth.