Nobi Tamako thought it was just a child's toy. She slotted the phone into the buckle on her belt and turned it clockwise until it lay horizontally.
In an instant, the room was flooded with a blazing red light.
"Eh——!!!"
A scream echoed through the entire house.
Nobita, still mid-dinner, hurriedly swallowed the food in his mouth, set down his bowl and chopsticks, and sprinted toward the stairs. At the same time, his father, Nobi Shinsuke, rushed out from the living room. The two locked eyes, exchanged a grim nod—as if preparing for death—and charged upstairs together.
There, on the second floor, stood a figure in armor—Faiz. The armored person wavered unsteadily, dazed and confused.
"Nobita, is this your toy?" the warrior asked, compound eyes glowing as they fixed on him. The voice trembled, but it wasn't pained. Nobita let out a sigh of relief.
"…Yes, it's my toy."
"These toys nowadays are really something," Tamako muttered in awe. She lifted her hand, feeling an odd lightness, as though limitless strength was coursing through her body.
"How do I take this off?"
"Just press the hang-up button on the phone," Nobita explained, pointing at the device attached to her belt.
But Tamako didn't move to remove it.
"Mom?"
"Wearing this makes housework so much easier."
"…But—"
Nobita's face twitched. He had spent so much time and effort making the belt, yet hadn't even used it himself—and now his mom wanted to keep it for chores? He felt crushed.
Through the glowing red eyes of her helmet, Tamako looked at her son and said in a choked voice, "Housework is exhausting. Can't Nobita be a little more considerate of his mother?"
What do you do when your mom is naturally overpowered?
Defeated, Nobita slumped his shoulders. "I understand. Please go ahead and use it."
"Nobita is such a good boy!"
Clasping her armored hands together, Tamako practically skipped downstairs. Even though her face was hidden by the helmet, it wasn't hard to imagine her radiant smile.
"…."
From the side, Shinsuke's eyes gleamed with envy. "Nobita, do you have another one of those toys?"
"You can't use it, Dad," Nobita muttered.
Unlike Tamako, who spent her days doing heavy housework, Shinsuke was just an office worker and part-time painter. He didn't exactly need superhuman strength.
"But… it looks so cool!" Shinsuke sighed wistfully.
Like father, like son—their eyes met, both sparkling with the same thought.
"Alright," Nobita said with a grin. "I'll make you one. But you'll help me assemble it."
Though he wasn't a scientist, Shinsuke's painter's hands were steady and meticulous—steadier than Nobita's, in fact. That night, father and son worked side by side, and before long, they successfully built the Kaixa and Delta belts.
By midnight, the entire Nobi family had gathered in the living room. Each held a transformation phone—Faiz, Kaixa, and Delta—and wore the matching belts around their waists. Their eyes shone with anticipation.
"The Nobi family transformation ceremony begins now!" Shinsuke declared proudly.
Together, they activated their phones.
"Standing by!"
Three identical electronic voices rang out.
"Henshin!"
They slid their phones into the belts.
"Complete!"
In an explosion of red, yellow, and blue light, three armored figures emerged—each different in height and color.
Though the suits boosted their stature, the difference between wearers was obvious: Nobita in Delta was still the shortest, Shinsuke in Kaixa stood in the middle, and Tamako in Faiz towered over them both.
"The Nobi family cleaning squad begins now!" Tamako announced heroically.
Despite their groans of protest, Nobita and Shinsuke gave in, joining her in housecleaning.
And so, the very first mission of this world's three Kamen Riders… was spring cleaning. Truly unprecedented.
Meanwhile, elsewhere in town, a glowing screen materialized. From it stepped a handsome man in a black suit, pink shirt, and a camera hanging from his neck. His expression was calm, indifferent.
"A Kamen Rider has appeared in this world too…" he muttered.
The next morning, life seemed ordinary once again.
Though Nobita had mastered technology capable of reshaping the world—perhaps even the universe—he still went to school like always. Not for study, but to enjoy the school atmosphere.
Only after entering society does one realize how precious school life really is.
Though physically still a child, mentally Nobita was already someone who had lived through adulthood. He knew how complicated the world could be, and so he cherished his current leisure deeply.
"Good morning~," he greeted as he entered class, sliding into his usual seat: second to last row, by the window.
Normally, given his height, such a spot wasn't his to take. But if he wanted the seat, no one dared stop him.
His deskmate was none other than Minamoto Shizuka.
"Good morning, Nobita!" she said brightly.
Today she wore her usual outfit—twin ponytails, a light pink shirt, and a blue skirt. Ordinary for an elementary school girl, but radiating pure youth and vitality.
"Good morning," Nobita replied calmly.
Shizuka pouted. "Honestly, Nobita, you're always so cold."
"That's just my personality," he shrugged.
The classroom gradually filled with students as the bell drew near. Among them came two boys, one fat and one thin: Gian and Suneo.
In this world, Gian was a heavyset otaku—less of a bully, more a gamer. Though cowardly and mediocre in studies, he was easygoing and got along with everyone.
Suneo, meanwhile, was still a rich kid, but low-key. He spent weekends at cram schools, picking up many talents, and remained Gian's close friend.
"Hey~," Nobita greeted them with a smile.
The two stiffened, forcing awkward smiles before quickly sitting far away to whisper to each other.
"Did we offend him lately?" Suneo muttered.
"No, I've just been going straight home after school," Gian replied.
"By the way, what've you been up to?"
"The limited edition Omegamon cards went on sale. I managed to pull one."
Suneo gasped in awe.
At that moment, another boy entered the classroom—Dekisugi.
In the original world, he was cheerful, helpful, and brilliant. But here, in this reversed world, he was still a top student—only expressionless, introverted, and unreadable.
He walked in, glanced briefly at Nobita, then silently took his seat.
Ding-dong—
The bell rang. But instead of their familiar homeroom teacher, Eiichiro Teraguchi, a tall, frivolous-looking young man strolled into the room.
"Huh?"
The class murmured in surprise.
The man leaned casually against the podium. "Your teacher is sick. I'll be filling in for the time being."
"Is this guy… really a teacher?"
Even Nobita frowned. Substitute or not, he should at least belong to the school staff. Who on earth was this man?
The stranger sat down on a chair, crossed his legs, and said lazily, "This period's self-study."
"Wait, are you really a teacher?" one girl asked.
"Of course. Isn't it obvious?" he smirked.
"…What's your name, sensei?"
"Kadoya. A passing-through Kamen Rider."
"Kamen Rider? That sounds strong!"
"Masked Rider is…"
Kadoya trailed off, exasperated. Even the Destroyer of Worlds was helpless before a class full of curious kids. After all, you can't punch children—and as a Kamen Rider, losing face like that was unthinkable.
In the back row, Nobita's glasses gleamed white.
Kadoya?!
A passing-through Kamen Rider… there was no doubt. This man was Kamen Rider Decade—the Destroyer of Worlds himself!
But this is the world of Doraemon. I haven't even used the Anywhere Phone Booth yet. How did another Rider get here? Could this be… a crossover world of anime? Nobita frowned, deep in thought.
Meanwhile, Kadoya scanned the classroom. If this world assigned me the role of substitute teacher, then the Kamen Rider of this world must be here too.
Most students were just curious kids. But five stood out: Gian and Suneo chatting about card games, Dekisugi sitting silently, Shizuka sneaking glances at Nobita, and Nobita himself, quietly observing.
Any one of these five could be the key, Kadoya mused.
But if there were Riders, there must also be monsters. Yet he had searched the whole town last night and found nothing unusual. Could this really just be an ordinary town? Impossible. Something was hiding beneath the surface.
"You five—come with me," Kadoya suddenly ordered. He led them to the rooftop.
"You really have no interest in Kamen Riders?"
Gian and Suneo's eyes lit up. "Kamen Rider? Is that like… Lion Mask?"
Kadoya frowned. It wasn't them. His gaze sharpened as it fell on Nobita and Dekisugi.
"If you won't reveal yourselves, then I'll just use my own method."
He produced a white camera-like device, fastening it to his waist. A belt and card holder extended from it. Drawing a card, he slid it in.
"Henshin!"
"Kamen Ride… Decade!"